It's simple. Nature got us wanting the alpha male. Bad people tend to match the title.So...
I would like to know this too....lol
I honestly believe it is just because very physically attractive, or otherwise desirable men get a lot of attention - they don't need to be nice to be successful, so its basically random luck for a girl if that guy is nice or not.
I also believe that women fall in love or become infatuated more often because of physical attraction than personality... or at least that physical attraction makes it much easier.
Ugly, average or even moderately attractive men have a much harder time with women and don't want to screw it up, which means they have pressure not to be bad guys. Combine that understanding with the jealousy, and the sentiment that women are often with bad men gets amplified some what... I don't really believe it is the norm, its just so annoying that it gets a lot of attention. Esp. when you know some d*ck guy who is with a really nice girl and you desperately want to save her but she hides behind 'love' as an excuse to stay with him...
I don't know we just do !!