I hope nobody minds if I ramble about a Bill headcanon for a bit….
Both times Bill has appeared before someone in canon, he presented them with a “gift”. Sounds like a good idea for a demon trying to win someone’s favor, convince them that he’s trustworthy enough to make a deal with, right? Except Bill’s “gifts” have always been something HORRIFYING, and he doesn’t even pretend not to know it. He just drops whatever disturbing object he summoned in front of the person, gives them a few seconds to freak out, and makes it go away without a second thought. Any other otherworldly demonic being might at least pretend to be offended that the person didn’t want his gift.
So why then? What’s the point of giving people things if you’re not going to even pretend to think they’d want to keep it? Certainly not to gain their good will. The answer, I think, is in the reactions of both Gideon and Dipper. Gideon fearfully accused him of being insane, (with which Bill casually agreed) and Dipper, if I remember correctly, just screamed. Dipper wanted nothing to do with this demon, but Gideon had intentionally summoned him, and both boys were frightened.
It’s a demonstration of power. He’s not just being some weird perverse kind of “friendly”, he’s going out of his way to frighten people immediately after his appearance. And why would Bill feel the need to do this?
To assure them that he is a threat, perhaps more so than he really is.
(hence the allusion to the illusion of the Shepherd’s Tone)