The only thing that I can think of that can "redeem" Sam is that he really WASN'T Dean's Sam anymore, and I'm not talking about the siren influence. He did have that demon blood in him after all, and a switch has been flipped somewhere, and he's already on his way *there* if not already there. I think the moment he used his powers in 'I know what you did -', the Sam that we knew is gone.