Dean didn't want to believe it. He didn't want to believe for one second that his brother was some sort of twisted monster feeding off of another twisted monster. The very thought made him want to be sick, and for a moment he was right there with Claire, wanting to say that Cas was wrong.
But it was the only conclusion that made sense.
No way else Sam would ever have been such an ass to all of them. Being a jerk to him and his Dad was one thing but Ben? Their Mom? That wasn't the Sammy Dean knew. Neither was how irritable he had been, how he came and went, how some days he acted like he was on top of the world and others he acted like a woman PMSing.It made sense. Had it been anyone else, Dean would have already found the way to off them and moved on.
But he wasn't going to off his brother. No, he was going to off the demon bitch responsible for all of this. He had threatened to kill Ruby before for a great many things, but now he wasn't in the mood to make threats. This had hurt their Mom and Ben, and it was killing Sammy...at least the part of Sammy that was his brother.
He lunged away from Claire without thinking, without threat or precurser. He was going to grab that bitch's knife, and when he did, while she was trapped in the trap, he was going to stab her again and again and again.
And again.
He was going to stab her until the anger faded...if it ever did.