Writing evil characters guide

Motivation is key

First and foremost your villain needs to have a realistic motivation to be evil. Villains who are evil purely for the sake of evil are unrealistic and this leads to bland and boring characters.
Let's take Hannibal Lecter as an example. Hannibal kills and eats people because this is the ultimate show of dominance, the ultimate way of showing he is in fact an apex predator and superior over all others. Hannibal sees himself as God's equal. This is his primary drive.

On top of that Hannibal is intellectually brilliant, he's sophisticated, well mannered and cultured. He's curious and has refined tastes in cuisine and other arts. This curiosity and his skills in cooking, along with the traumatic event in his childhood where he witnessed people eat his beloved sister are also part of why he kills and eats people. Since the men who ate his sister were crude and rude he became a sophisticated and cultured man who absolutely despises rudeness, so much so that some people have paid with their lives for their rudeness.
To some extent, and depending on which version of Hannibal you're looking at, killing people is also a test to Hannibal, a test to see whether or not he can get away with it. He keeps increasing the stakes and he keeps getting away with it.

Childhood trauma is a common element in the backstories of villains, but it can quickly come across as cliche, especially today. A childhood trauma alone isn't a basis for an evil person, but it can definitely be a start. There still has to be a reason why this trauma lead to this person becoming evil though, as traumas could just as well lead to a subdued character or simply a heavily suppressed memory.

Psychopaths and sociopaths

Psychopaths and sociopaths often make for great villains as they're inherently different from us and usually in a darker way. Perhaps less so in the case of sociopaths and not all sociopaths and psychopaths are criminals, evil or dangerous in any way, but their archetype can make a solid foundation for an evil character nonetheless.
But first, what's the difference? If we focus on the more stereotypical versions, both psychopaths and sociopaths are unable to empathize with others like regular people and both are usually the result of faulty brain regions, but psychopaths are the more violent types and the ones without a sense of right and wrong, while sociopaths often do have some sense of right and wrong however skewed their version may be.
As far as committing crimes go a sociopath will generally do so erratically and based on impulses, while a psychopath will carefully plan their crimes in advance and take calculate risks.

There's far more to this of course, but those details'd require a far more in depth guide. The main point is that both these archetypes work great and are worth investigating when writing a villain. One reason why they work so well is because both psychopaths and sociopaths are often charming and capable of manipulating the feelings of others because they don't feel feelings like regular people do.
This alone makes for an interesting villain to some extent as the hidden evil behind the charismatic persona opens up a wide range of opportunities for tension, reveal and other elements to play around with in a story. Plus by building upon the stereotypical psychopath or sociopath you potentially create a unique and interesting character, like Hannibal Lecter.

They're people too

It's important to remember that your villain is a person too. You might not see a serial killer as human, but they are, or arguably were, people too. In the vast majority of cases a villain is only a villain because something changed them in life. It could've been a gradual change or they could've snapped, but at one point they were like any other person. By humanizing your villains you make them easier to relate to on some levels and thus more interesting. It'll be the difference between a common slasher movie villain simply out to kill people and the ruthless serial killer driven to insanity by a tragic event or misguided belief.

Villains being people too goes a step further though. Villains are often overlooked when it comes to character progression, but character progression helps make characters more realistic. People change, it doesn't matter whether that person is good or evil. Allow your villains to change based on what happens to them, if that ultimately means they change from a villain to a hero that's great too. Some of the best characters started out as villains and became heroes and vice versa.

Hero of their own story

"Villains are the heroes of their own story" is a phrase commonly thrown around when people talk about writing great villains and to some extent this definitely holds true. Hannibal Lecter thinks he's better than everybody else and therefore he's the hero in his own story. This point ties in heavily with the psychopath and sociopath point, as it's about having a misguided moral compass, but you don't have to be a psychopath or sociopath to have a skewed moral compass.
We see this in various ways in our every day lives too, politicians who lie for personal gain. A criminal stealing from innocent people or selling drugs to others simply because they have to do something to make a living and survive.

By taking this and taking it to an extreme you quickly create a villain. In fact this is often part of the origin of a villain throughout their life. However, simply having a different view on life or doing something illegal doesn't automatically mean you're a villain or evil in any way. Fighting for what you believe or fighting to survive isn't evil, it's the way you fight and survive that can make you evil.
For example, fighting for a united world, one where peace is the norm is very admirable, right? Well, what if that person fights for this world by killing all those they deem unworthy? It doesn't even have to be a race, gender or age related basis either, they could simply be killing all those responsible for evil in our world. Of course you now thread on the line of whether they really are evil or not, something Dexter played around with as well for example.

True to life, and what works on paper

There are, unfortunately, countless stories of real life people who committed acts of sometimes unimaginable evil. Surely they'd make for great sources right? Well, not exactly. Not everything that happens in real life works well on paper, so sometimes you'll have to alter the way your characters behave to make them more believable and interesting, even if that means making them less like people in real life. It's counter intuitive, but it's definitely something that could make or break a story.

For example, many criminals have either been caught or managed to escape thanks to incredible luck, stupid mistakes or other unforeseen circumstances. If you use these in a story it'll likely come across as cheap, unrealistic and as a cop out.
Some real life people are also just so completely different that basing a fictional character on them can make it very difficult or even impossible to make that story convincing.

Writing and developing evil

So far I've gone through quite a bit already in terms of what makes a great villain, but you might be looking for specific ways to translate all this in a story. Well, fear not as I will go into such points now.

Witness evil

Make your character do evil things, preferably to characters we care about. This may seem obvious, but somehow this is still overlooked at times. Simply stating a character is evil isn't enough. It also matters what the character does and to whom.
For example, you might have a character that has ordered the destruction and annihilation of a nation and all its inhabitants, but the character will appear far more evil if that character took part in it. It's even more effective if this character performed some evil deed against characters we love.

Take Dolores Umbridge for example and let's compare her to Voldemort, or at least the Voldemort we know up until and throughout the Order of the Phoenix, which is when we first meet Dolores Umbridge. Voldemort is obviously the big bad guy in the stories, but up until this the most personal harm he has done to Harry is kill his parents, which was revealed in a point of the story where we knew very little about both Harry and Harry's parents, so there wasn't a whole lot of emotional involvement for the reader. It's horrible and evil, yes, but Dolores definitely has a far more emotional impact.

What did Dolores do? Well, ignoring her incompetence as a teacher she started of with torturing Harry Potter as a punishment by making him write "I must not tell lies" using a Black Quill, which carved what he wrote in his arm. This alone is enough to make a reader hate a character instantly of course, but it continued. She started to gain power within Hogwarts to the point of becoming headmistress. She fired teachers she deemed unfit, often for racist (or I suppose speciest) reasons. She tried to block Harry's every move, banned him from Quidditch, a game he loves so dearly and even tried to expel him from Hogwarts.
Does this compare to killing Harry's parents? Of course not, but it still has a far more emotional impact on the reader and induces far more hatred, which is usually the entire point of a great villain.

Counter the hero

A great villain is often one who counters the hero. This doesn't have to be a case of the hero wants X and the villain wants Y, both the villain and the hero could want X, but they just try to achieve it in different ways. To bring back my earlier example of desiring world peace, this could be something both the hero and villain are fighting for, but since the villain is fighting for it through terrible means the villain will have to be stopped.

A great hero counter example is the way the Joker counters Batman. Batman fights for justice and wishes to clean up Gotham and does so using a strict moral code, like refusing to kill anybody (depending on the comics). The Joker wishes to make Batman break his own moral code, so much so that the Joker would love to be killed by Batman. The Joker's entire life is dedicated to playing and messing with Batman, but they both refuse to kill each other. Batman refuses because of his moral code and the Joker refuses because then the fun would end.

Viewpoint character

A very effective, but not always viable option is to make the evil character a viewpoint character. Whenever a character is a viewpoint character you're bound to learn more about why they do what they do. If the reason behind it is understandable to some degree they'll become easier to relate to or at least more human and thus more compelling. It's why people might root for characters like Walter White, Hannibal or Jamie Lannister for example.

Love, compassion, and vulnerabilities

A great way of humanizing an evil character is by showing they can feel love as well. This is perhaps less relevant for psychopaths and sociopaths, but it definitely works for other types of characters. Just because a character is evil doesn't mean they don't feel love or compassion towards something or somebody. Family, partners and friends are present in even the lives of dictators, plus it leaves them vulnerable and vulnerabilities are humanizing as well.
By leaving them vulnerable I don't mean their family should be taken hostage or anything similar of course, that's what a villain would do, not a hero. But if something were to befall their loved ones the villain would obviously feel horrible and this makes the reader feel sorry for them or take pity. Which makes the villain more compelling.

On the other end of the spectrum you have sociopaths and psychopaths who are incapable of feeling love, at least in the way regular people do, and you have those who are abusive in a relationship, like the Joker with Harley Quinn. Any tender moments there might be are contrasted by horrifying abuse, which further establishes a villain as evil. But it can be tricky to not go over the top with this.

Permanent damage

The best villains are often those that are capable of doing permanent damage. A threat of permanent damage, like the destruction of the world can make for an exciting story, but things get far more thrilling when the threat is more realistic and possibly more personal. If the threat is actually executed it's even better, as far as making a character convincingly evil anyway, but it's definitely not a necessity.

The Joker is another great example of this, esepcially when you compare him to some of the other villains in that same universe. The Joker has managed to maim, kill and otherwise do permanent damage to Batman and his allies. Something that can't be said for all villains Batman has faced.

Mystery evil

In some cases the evil character isn't revealed until much later, but you'll still want a threat to be present. In these cases it usually works best to have a representative force or some form of henchmen working for the cause of the evil character. By henchmen I don't mean the literal goons and expendable armies of soldiers, but serious threats to the hero. This could even tie in with the love and compassion angle from earlier, depending on who the main evil character and the henchman is.

In Harry Potter Voldemort takes a while to really show up, but in the meantime there are plenty of threats to fight and Harry even has to battle Voldemort in his own mind due to the connection they have.
In the Lord of the Rings Sauron is basically just a floating eye, but the powers in the ring make his threat real and frightening. This is definitely another case of the 'witness evil' point though, as the ring's far more terrifying and hated by readers and viewers than Sauron.

Final words

Hopefully everything so far has been helpful to you, but the main thing to take away is that evil characters are characters and people too, so treat them like you'd write any character by making sure they're complex, developed and respond to their world in a realistic manner. As horrible as they may be, they still have their desires, dreams, goals and fears. Without them a character feels two dimensional, no matter whether they're a hero or villain.