I believe the Hunter is in fact a fundie geek, who tries to seperate "true" geeks from mainstream geeks.
Probably he is from a secret geek society, sent to find out whether the weregeeks can be trusted to not abolish their geekhood under pressure.
He drove Mark right into their meeting, so it's not unreasonable he wanted him to participate in their LARP.
The knife? Uh. Maybe he is really good at throwing knives and aimed to miss him? Some sort of obscure knife geek, probably. (Never heard of such a thing. But never heard of a geek hunter either.)
Maybe Joel knows about his background, maybe not.