Derek would have been surprised, and maybe even a little guilty, if Stiles had actually been frightened by his threat. As it is, though, he's too focused on the kid's last question to even process the non-reaction. And then too focused on Stiles enumeration of the many and varied ways in which Peter might--and has--screw him over. He has an almost knee-jerk desire to tell Stiles that Peter's getting better, except he's pretty sure it's just wishful thinking feeding on his loneliness, frustration, and desperation.

"He said they're here because they want me to join them." He'd been assuming they wanted to kill him. Challenge him for his territory--not for his pack, obviously, it's not really even big enough to qualify. But if they want him to join up.... He squeezes his eyes shut. There's no trusting Peter. None.