Since im playing Injustice right now all i can think of is Superman, the reality of someone with all that power letting loose is both terrifying and exhilarating! Batman going bad would be wrong for his character, i can maybe see him going a little crazy but turning actively bad just wouldnt feel right, i dont think i can explain why but Batman has to be the good guy.
Marvel on the other hand have been switching their characters from good to evil and back again for years, Jean Grey, Professor X, Cyclops and Angel have all gone bad at some point, Magneto, Emma Frost, Apocalypse have all made the switch to good. I have to say out of all the marvel folks that turned, my favourite was Archangel during the dark angel saga over on X Force, his transformation happend a tiny bit at a time and right under everyone's noses. I also really enjoyed the evil Deadpool in Deadpool kills the marvel universe, behind all the humour and comedy there is a very dangerous killer, its easy to forget when he is so goofy but the guy is insane. Honerable mention goes to Ultimate Reed Richards, the guy makes Dr Doom look like a saint.