Dean had gone feral because of the mark and the killing.. It was worse when Castiel appeared.

Castiel and Sam were trying to remove the mark from Dean, to prevent him from losing Control and when they did.. Dean just ran away, and sadly for them: Dean went to kill every single of the cult that killed Charlie.. That only feed the mark even more, more blood seeking feelings.

It was just after the intense fight that Dean regained a bit of control over himself, and he found himself pinning Castiel down to the ground and holding an angel blade, which was halfway of stabbing Castiel.. This was not good. Dean's break hitched as he looked at his "friend's" eyes with pure confusion, rage and clarity. Castiel was beaten up, blood coming from his nose and mouth as his breath was heavy and slow, he wasn't gonna die but still.. All the times that Dean had slammed his head against the wall or the table of the bunker really payed the price, and he was fucked up. Dean frozen as he realized what he was about to do before regaining control over his actions: he was about to kill one fo the people that he considered family or.. Just close.