People are complicated and we often only see the tiniest fraction of who another person is. Stories are one of the ways we can see deeper into lives beyond our own, which is one of the reasons why it gets so infuriating when those stories are told by flat characters who never manage to come to life. In a way, it almost feels like a cheat: like being promised gold and being delivered a cheap, gilded knockoff.
In real life, people are a mixture. Even appalling people often have endearing traits (and some of the most effective horrible people are able to hide their evil behind hypnotic charisma) and even outward saints have some secret sins. So why do we keep finding characters who are one- and two-dimensional?
My personal theory is that it's partly because people like simplicity. To paraphrase JMS: we want to cheer for the good guys, boo the bad guys, and enjoy the satisfactory thump when the villain hits the floor. In order to do that, we need to be able to tell right away who is the hero and who is the villain.
Another part is that we don't like the idea of "bad" people getting good things. It offends our inherent sense of fairness. I think this is why we have a lot of pushback against unlikeable heroes (and particularly unlikeable heroines). And even though most people enjoy a redemption story, they want to root for a good guy who has been misunderstood or been caught up in bad circumstances, but not a bad guy who somehow stumbles into a better situation.
Everyone has a short list of things that are absolutely unforgivable in a character. Personally, I can't get behind a character who denigrates the vulnerable, particularly if they use slurs. But it would be interesting to see a story where such a person starts to recognize why that's a problem, recognizes the hurt they've caused, and then demonstrates real growth and change. However, that character could not be a romantic hero or heroine with an arc over a single book. I would need to see improvements before they got their own story. (If I were to plot such a story, the character would be a secondary one who is an initial jerk but learns the error of their ways before the end of one book, then they would have a chance to demonstrate that they are now a better person in their own book.)
This is one of the areas that I think authors overlook sometimes. We are often exhorted not to use unlikeable characters because we don't want to alienate readers. But it's also boring to read about characters with no flaws.
Books have a rare opportunity to allow us to see the full complexity of a character. We don't have to guess at what they're thinking and feeling, we can be a part of their experience. And maybe, it can help us to be less simplistic in our own judgments of others.
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