15. Mums Can't Be Trusted
This is a rule that has been instilled in slasher films since before the slasher genre even existed. Norman Bates, anyone? Yes, that sweet little old lady that you met early on in the film might seem kind and generous. She might even have offered you some milk and cookies, which you chuckled at as you thought about the night of hedonism ahead. But chances are she was actually driven insane when her unattended son Jason drowned in Camp Crystal Lake, and is actually looking forward to feasting on your internal organs later on in the film.