Why is it that when we grow older and re-watch the cartoons we loved, we find ourselves loving the characters we used to hate as children?
Key example: Danny Phantom and Harry Potter. Excellent series, both of which I LOVED the main characters and had minor crushes on (more Danny than Harry). Then came the villains of the story. Draco Malfoy annoyed me to no end, I never really getting my younger sister's crush on him until I was older, and I absolutely HATED Vlad Masters aka Vlad Plasmius. They were bad guys that constantly got in the hero's way.
Now that I'm older, I still find Danny Fenton amusing and Harry endearing, but I find myself liking Draco and Vlad Masters/Plasmius more. Even now, with Teen Titans, I find myself loving Slade (Slade Wilson aka Deathstroke for the comic lovers), a man who I absolutely HATED because of his drama filled involvement with not only Robin, but Terra and Raven as well. He was a brilliant dastardly villain who was five steps ahead of everyone and knew how to get under the characters' (*cough* Robin *cough*) skin, and I despised him for it. Nowadays, after rewatching the series, I find him such a brilliant bastard, I can't help but admire him for it.
If any rate, I find myself liking more anti-heroes than bad guys (or good guys), because essentially, it's very hard in the real world to have a straight, black-and-white good guy and bad guy. Anti heroes have more realism, more humanity, more flaws than heroes or villains combined. Actually, that's exactly what an anti-hero is: neither the hero nor the villain.
So why is it that I like these characters? Is it because I sympathize with the bad guys, because they're only bad because of circumstances took them down a wrong path (Hell is paved with good intentions)? Or is it because I have changed as a person, and I wasn't the "can-do" aspiring little superhero-syndromed child anymore? That I see more realism in characters and connect better with the ones that daily have struggles we have already faced in some way or another?
We like to see our heroes have flaws, to struggle against all odds. It's boring to see things come easy to a hero; it's unrealistic. It's why we hate Mary Sues so much: they've got it too easy.
I think it's why we like antiheroes so much. They are the bug spray to Mary Sues. They have flaw and character that we can relate to our own struggles, our fight for change. They are fictional characters done right. If anything, they are almost human because of it.