Go to youtube and search for it, they might have it.
What I remember was that Sam and Dean went to investigate a town filled with demons that were possessing people. But It wasn't really a demon case. It turned out to be this angel that was making people think they were demons, but they really weren't. He made them kill each other. When Sam kills one of the "demons" which he thinks is a demon but really isn't. He stares at the blood on the knife. Dean finds him, and in the end, they end up fighting again. Sam suggests that they both part ways, because he says he's too dangerous to be around Dean. Dean agrees, and they haven't seen each other since. Sam has tried to give up hunting, while Dean is still out there doing the job.
Hope i helped. =]
