I pin Rusty down as one of those archetypal bullies from old Saturday morning cartoons: burly, but dumb and easily thwarted. Even after the Jack incident, in a world of demon children, werewolves, and other powerful beings, he's harmless.
And I'm going to guess he's gonna get checked in about a page or two, but man, maybe he knows what he's doing. A real life bully knows how to push buttons and get away with their actions, especially in school grounds. It's all about provoking and not being seen, in most cases, causing less commotion on your side. Another factor is knowing that your victim have more to lose than themselves (like academic reputation, for example.) If Dam freaks out, he might risk his entire stay on Earth.
Maybe Rusty doesn't know about the deal, but maybe he knows that Dam won't lash out. When he does, people are going to notice the Antichrist's unholy presence before the real bully. He's prodding the keeper of abominable horrors because he can.
And that's what gets him off!