I get the bad feeling that he won't get his proper comeuppance. They showed a few panels of the mastermind and the fat kid at the very same festival that Maria and douche nozzle are nearby (due to the fireworks display) and mastermind kid was looking at a message from the neanderthal with a concerned look on his face. Why would they show this unless it was to put the thought in the reader's head that they are close.
I predict mastermind kid will end up running into the asshole psycho right as he is dying and he'll try to tell him who was behind all the previous deaths but he'll call her "Nagare's mom" instead of referring to her as Maria-sensei so her identity will still be a "secret". Her mistake is trying way too hard to get catharsis by watching them suffer, she should have just offed him immediately and been done with it since it's too far out of her control at this point. The more she lingers the more likely she is to slip up and the only thing torturing them really does is (maybe) make herself feel better since they won't give a shit once they're dead.