It isn't easy being a slasher. In one scene Leslie details the excruciating process of finding the right group of victims. In order for it to work flawlessly, he's gotta have the perfect mix of jocks, stoners, and hormone-addled teens surrounding the virginal heroine who may or may not make it to the very end. In another scene he shows the fictional filmmakers how he sabotages the makeshift weapons and escape routes his victims might use to thwart him.
So it's a head-scratcher that the movie eventually descends into the very thing it's lampooning. There's a major reveal you'll see coming from several minutes away and although the movie thinks its uncharacteristically serious climax is subverting the cliches of previous horror films, it still very much feels like a run-of-the-mill finale. Since it's so creative for the first hour or so, the last ten minutes or so are kind of disappointing.