So why is your character a jerk? Not how, but why?
Example: Bad Fairy. Character: Delia.
Delia is my protagonist and she's perfect to do this exercise with because she is, after all, the bad fairy. Let's take a look:
How is she a jerk?
Most notably, she's arrogant. A bit elitist. Privileged and unaware of it. Sometimes selfish. And unfriendly to most people.
Why is she a jerk?
This is what's most important. That's a LOT of negative traits to saddle a child protagonist with. How do you write someone like this and not make them entirely unlikable? The trick is to show WHY a character has negative traits--and while you shouldn't go out of your way to "justify" the traits, you should allow enough perspective for the reader to understand why a person in the situation your character is in would feel and act the way they do.
For the record, though Delia is in essence a fairy tale villain and I'm writing about her childhood, she is not supposed to be a character you "love to hate." You're not supposed to hate her--and I've found that despite most of my readers connecting and sympathizing with her, I have gotten occasional feedback from readers who reacted to her very much the way her enemies did. That will happen if your character is a realistic jerk. Not everyone will like them.
So . . . why? Story-wise, why is Delia arrogant, elitist, privileged and unaware, selfish, and unfriendly?
- The arrogance, quite frankly, comes from her status as a prodigy. Everything is easy (or at least, easier than it "should" be), and when she sees students twice her age struggling to keep up with her on tasks that aren't much of a challenge for her, she develops an ego problem. But because others sometimes make their jealousy and resentment clear and try to shame her for it, the arrogance is her immature way of refusing to apologize for excellence.
- The elitism comes from comparing herself to her classmates and seeing that she is in a different league, and she has trouble respecting people who just don't care about the study subjects. But this is fueled by an absolutely consuming passion for the topics at hand, and she finds it almost offensive that her classmates pursue their education more as a vocation than an art.
- The unchecked privilege is at least nothing unique to her in the story. Her family and fairy school classmates are also part of a privileged class, as fairies are valued for their magical talents and always paid well, considered a sought-after commodity by the upper class of their society and included as a weird kind of privileged servants. However, as a person with human heritage as well, Delia is sometimes offended when she's assumed human and treated as such, and she later learns a LOT more about what not to take for granted, as well as develops more sympathy for those less fortunate.
- The selfishness is not always obvious, especially since she likes teaching and tutoring others or helping them with things they cannot do themselves, but when she's selfish, it's usually in the context of expecting others to cater to her because of her unique talents. But since so many of her peers and even her teachers have been hostile to her pursuits and have tried to roadblock her and sabotage her, she sometimes feels she must be wary of being too generous out of self-preservation.
- The unfriendliness comes from an obvious source: People aren't friendly to her, so why should she be friendly to them? It's more complicated, of course: She is standoffish, shy, and often read as creepy because of the child prodigy thing (plus her aesthetic tastes run "creepy" too), so she isn't very approachable. But on top of that, she is quite frankly obsessed with her studies, which makes it hard to put social connections in the forefront of her mind, and she has very little in common with her classmates because she's so much younger, so she finds her own company more satisfying.
Every one of these negative traits is complicated. People aren't just jerks. Giving them unlikable aspects is not about grafting them on and making them vain or clumsy or standoffish just for the hell of it. They've got to be vain because it's clear they value the superficial due to lack of talent, or clumsy because they're too inwardly focused to pay attention to where they're walking, or standoffish because they have been burned by friends before. Your characters, if they're going to have negative qualities, still need to think they're right. They have a perspective on what they're doing, and that perspective is formed by what they know and what they don't know.
And if being a jerk causes them enough pain that it prevents them from getting what they want, they have to be able to repent and grow. These traits aren't immutable aspects of their character, and they have to be written as though they formed the way we all construct our psyches in the real world. Our attitudes are the amalgamation of our experiences, interactions with others, personal values, and beliefs. If you don't know why a character acts a certain way or believes a certain thing, you might need to do some homework. Because that hollow trait will make it seem like your character wasn't alive until you started writing about them. Everyone has to come to the page with a past and a full set of attitudes toward their experiences. If you've balanced your character with negative traits like a math equation, the superficial construction will be visible in your final product.
So ask your characters why they're jerks. They'll probably be able tell you.