It was all that Dean could do to not come rushing out of his hiding place from the first moment that Azazel struck out at her. Dean knew full well that she was still recovering from Lilith, and he didn't want to see the yellow eyed bastard add so much as a single short cut or additional bruise to her battered body, let alone something worse. It took all of his willpower to not go storming onto the scene at the first hint of violence, but somehow he did, knowing this had to be believable. Were there no other backup, Dean would have been forced to wait, to hope for a better opening. He watched, hand clenched, as Azazel struck her, flung her around like a rag doll. He waited until he could wait no more, until the serious damage was about to start, and then he gave in.
Dean rushed out of his hiding place and drew his gun, which clearly wasn't the colt. His jaw clenched. "You fucking touch her again I'll fucking kill you.' He was hoping that his words, which were obviously idle threats because Dean had no way of killing Azazel with his handgun, would be enough to distract the demon from hurting Jo, or perhaps even enough to give his father the sort of opening he needed.