Derek's last slave tried to kill him in his sleep. So even if Stiles did try it wouldn't shock him. He was expecting it.

"There are less of us, yes. But we're not anyone's slaves. They couldn't keep us. We're stronger, faster, unpredictable. We can smell your emotions. We can hear your heartbeats. Know when you're lying. When you're nervous. Find your weaknesses. No humans would be able to hold us as slaves," he explains. "It's a fun world to think about though."