OK, I'm a guy, but I'm gay, so let me give my perspective...lol.
The formula of the "bad boy" is that he acts like he doesn't really want you. He doesn't let you feel "safe," like you "have him." Because, you know, that's the true key to a guy's attractiveness - unavailability. That's why girls always want to land that type of guy. It's like an ego boost to think that you've tamed the wild beast. And if, instead, he rejects you, he's all the more attractive, because it taps into that whole "What's wrong with me?" thing. You feel that if you can turn him around, you'll have really proved something.
I'm sure many women won't agree with this, but I think there's some basic truth here.