Movies are supposed to be fun. You're supposed to laugh, smile, and talk about them with your friends afterwards. Halfway through this movie I couldn't tell If I was watching a movie that was taking itself seriously and doing a bad job, or a movie that wanted me to be in on the joke. And then... well, if you've seen it, you know what happens. And suddenly you're laughing along with the absurdity of it all.
Honestly, would you have rather it turn out to be the same boring uninspired serial killer or haunted house twist that anyone can write? The people who hated this movie are the people who wanted a movie that fit all their preconceptions and went in the same direction we've seen a hundred times before. James Wan lambasted the formula, took a risk, had fun with it, and it paid off. This is what movies should be. Weird. Fun. Make more weird fun movies.