Derek hears the hesitation, and it's not really a surprise. He knows Stiles doesn't consider himself his. He's just helping him because he won't leave Boyd and Erica twisting in the wind, or risk that the Alphas will hurt other people he cares about. It's still hard, though... but not as hard as listening to him go on, catching his scent when, of fucking course, he figures out the rest of what Peter said on his own.

"At least... if Peter's right, at least we know it's not just wishful thinking and they're still alive." Because it won't accomplish what the Alphas want unless he kills them himself. It's not all that reassuring.