It's less that the psychotic murderer in the movie behaves like a psychotic murderer and more that the people making the movie are themselves kinda behaving like psychotic murderers in the way they draw out, aestheticize, and hyperfocus on all the psychotic murder happening on screen.
Seriously, you can't watch the Terrifier movies (especially 2 and 3) and in good faith act like the director isn't taking pleasure in what he's depicting. And for the record, that is in absolutely no way a judgement against him, the movie, or anyone who likes him/the movie. Enjoying Terrifier is not an endorsement of extreme violence or psychotic murderers; neither is making a Terrifier movie. But also, let's not be obtuse here.
FWIW, I think the Terrifier movies are extremely stupid and shallow, but I think it would be a fun fit for the event. It has the right kind of grand guignol spirit.