It's an interesting situation. In order to get what he wanted, he needed Argent to trust him. When he had proven to be the least trustworthy person. But, for some reason, Peter didn't want to run. He was tired of it. Running, fighting, scheming, and nothing to show for it.

"I give you my word. I won't try anything."

So, he moved toward Argent, dipping his head to make it easier for Argent to reach the lock on the back of the wolfsbane collar.