All Stiles could see was red. Who did Derek think he was? He wasn't his alpha. His inner vampire accepted no master. In fact, he felt he was the alpha here - strictly speaking. It was only his human half that told him to back down or Derek might use him as a chew toy.

"Yeah well, don't talk about my father like you know him better then I do. I don't talk about your family." Except his creeper uncle of course.

Of course he wouldn't attack Derek from behind. It never even crossed his mind.