Confession--I like unlikable characters. A lot. A lot a lot.
Don't get me wrong. I love me some Atticus Finch. But his kind of morality is something that mere mortals like me can only aspire to with the knowledge that they probably won't ever live up.
Nope. Give me someone unlikable--a Scarlett, a Rhett, a Nick and Amy, even a Humbert Humbert--and I'll be happy.
What's there to like about unlikable characters, you might ask?
They tend to be more complex than other characters. They have a unique moral compass, which sometimes points towards good, sometimes towards bad, and sometimes towards self-interested. We'll go with a spoiler from Gone Girl. Why would Nick and Amy decide to stay together? Because they're unlikable characters who have their own unique reasons for doing things. Nick stays because he feels like it's responsibility, but he also can't quite help himself; Amy stays because Nick kind of returned to the asshole she fell in love with. Think about it--how often do you (or someone you know, because we're far better at judging other people's actions than our own) do something that makes absolutely no sense to anyone else, but you have you own very good reason? Quite often, I would imagine. Doesn't make you bad. It makes your decision unlikable.
They have a wider range. I think this makes them more interesting. You never quite know what they're going to do. They might do something good or they might do something bad or they might go with a third option. But you can't really guess what they're going to do. The good characters, the likable ones, such as Melanie, you know what they're going to do.
They tend to be more fascinating. Let's take Humbert Humbert. Pedophile! Yet we continue to read Lolita, a novel told from the perspective of the pedophile as he walks us through his lust and acting on his lust. His actions are utterly despicable, but his analysis and insights into himself truly fascinate me. I can't help but being drawn in by his storytelling. I don't know much about psychology, but I think if I did, I would love to analyze him.
Lastly, I think unlikable characters are more realistic. People can change, yes. But the kind of 180 transformation so often portrayed in movies and books isn't realistic. People struggle to be good all the time. And if someone does make a radical change to themselves, I can pretty much guarantee they will struggle for goodness somehow. An article I read not too long ago was talking about Seinfeld being a show without some sort of moral epiphany at the end. These characters weren't going to become good at the end of an episode. They were going to remain the same unlikable people they were at the end. I don't care for the show, but I do appreciate that about it. The characters keep it real. They are who they are and you can take them or leave them--just like real people. Everybody who's interesting to be around or talk to has a vice of some kind, in my opinion.
Of course, all this might really do is prove that I'm an unlikable character myself...