Adrenaline jolts through him when he hears Peter chasing after him, the teen running full force toward the house. He knows Peter is much faster than he is, and really that's the point, knowing the werewolf is going to catch him. Likely before he even sets foot on the front porch.

But then again, that's half the fun.

They're both worked up from the fight (and the kill, though he's in deep denial about that himself) and he knows this chase will help them both in the long run.

When Peter is close enough, when Stiles knows he's going to lose this chase, he tosses the bag off to the side so when Peter inevitably catches him nothing will be broken or ruined.