User blog comment:Rafe Adler/"The Thing" Discussion/@comment-176.221.122.115-20180406125535/@comment-3169324-20180406141416

Demons are just (twisted) human souls. If human souls can be used to power an angel's grace, then the reverse might also be possible. We've already seen something like this with Lily Sunder, although she was still a normal human.

And keep in mind, if Asmodeus only takes a small amount of grace at a time, it'll be restored. Why kill the golden goose? And it definitely did seem like he was addicted, not unlike Crowley was to human blood.