There's always been a hint of something bad in Rick's fellow cop, Shane, but (HUGE SPOILER ALERT if you haven't watched the Oct. 30th episode) he has helped lead the group up til now, and he's a helluva shot. However, the ending scene revealed a darker side when a flashback shows Shane basically sacrificing sweet, fat Otis in order to escape the Walkers.
Some would argue that Shane was just doing what was necessary to survive. Otis was holding him back by being slow, and Shane cared more about Carl (his pseudo-son) than the man who accidentally shot him. But it was heartbreaking to see Otis fight back, with the desperate look on his face as he came to realize that Shane was betraying him...leaving him for bait.
Perhaps Shane will come into his own as a bad guy. The viewer has even less reason to trust him (in addition to that whole sleeping with Rick's wife thing.) Even if he goes over to the dark side, Shane still represents a former good guy--leading to the question: how would I react if I was in survival mode? Would I take down the nice old guy who might end up getting me eaten? Would I be like Andrea and decide suicide is the best option? Or would I be like Rick and insist that the world still holds some kind of beauty?
Even though the show seems like it's all about the dead, the series is really about the living and how people change when they're faced with undeniably sucky circumstances.