[ And he knows this. Trouble is, William's real concern is that this demon could take advantage of Grell - pull him into some bad situation and hurt him. Or even just decide he wants nothing to do with him anymore and try to dispose of him, how Sebastian almost did before William interjected.

...William just doesn't see that he's phrasing his thoughts entirely wrong. ]

What do you suppose we do then, Knox? Wait patiently and see what happens? If your theory is correct, then there is also the possibility that demons from this world could be worse than our own.