My guess was he is pretty much a psychopath, kinda like Stu. Which gave him the fun idea of remaking the original killing spree, though Jill's motive must've appealed to him especially seeing as he already blogs online and is known for the "Stab-a-thons" (There totally should be a "Scream-a-thon") and the high school's film club, that and he kinda loves Jill and maybe something what Billy said in the first film. "Maybe your movie freaked mind lost its reality button, you ever think of that?"
"Be careful, this is the moment where the supposedly dead killer comes back to life for one last scare"
Jill's motive was pretty clear. Jealousy. Charlie was the love interest in this case who saw it as an opportunity to get in on all the action. Plus being Jill's deranged boyfriend didn't hurt matters at all.
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