Board Thread:Legitimate Canon Questions/@comment-122.109.62.213-20140729092340/@comment-13965604-20140805040808

See, Sean, the problem with your theory is that it takes a while for the wolfsbane to kill, but that's because it takes time to work it's way to the heart. That's why Derek wanted Stiles to cut his arm off. But Peter was hit darn near his heart, if not in the heart. It wouldn't take long for the wolfsbane to make it the rest of the way.