Wait a minute...
Just realized that the author is the one who made Kiriko and Kiriko Kill. And Hakaijuu too. Kiriko featured kind of a similar slasher premise with the monster killing off everyone except the MC who turned out to be the one most directly responsible for its resentment in the first place. But the sequel then went balls out crazy with the monster essentially becoming the MC's gigantic pet who he then utilizes in his plan to destroy the world, starting with the Diet building. I'm not even joking.