certainthings

Dean does know he broke the first seal, so I think you're right that part of his anger is really at himself and not Sam, but I think the anger towards Sam isn't necessarily "You started the apocalypse, asshole!" but rather, "You listened to a DEMON over YOUR OWN BROTHER." In which case, Dean really does have every right to be pissed as hell at Sam. That is what I have interpreted his anger as being about, anyway. Not the apocalypse but just Sam choosing Ruby and listening to Ruby over Dean, given the fact that Dean has basically spent his entire life trying to protect Sam and give him what he wanted, not to mention y'know, going to hell for him, and for Sam to listen to the word of a demon over Dean was more than adequate reason for Dean to be pissed off.