The Dumbest thing Someone Does (SPOILERs I'm Sure)
1. The worst is that everyone tries to shoot Michael. It's obvious from his history that bullets, even a coat-hanger in the eye have no permanent effect on him. Laurie and Sartain for sure should've known. I'd guess that Laurie would've passed that on to Karen, too. But noooo... !
The fire was the first killing method I noticed that >> might << have worked. Of course we heard after the credits that it didn't.
2. Laurie leaving the hiding place, then everyone else leaving it. I'll put Ray's advancing toward the danger here too.
3. General waving-around of guns with no regard to where they're pointed. Laurie, at least, should've known better. Seems like so many film-makers do not.
It seems the only smart one in the movie was that annoying baby-sat kid. He reminded me of Rodney Allen Rippy, the Jack in the Box kid of their 70s commercials. Probably tested as "cute" in several focus groups.