Happy Birthday to Me (1981)

Virginia (Melissa Sue Anderson) is not the killer, even though she finally remembers that her murdered friends abandoned her birthday party several years prior. That indirectly caused her mother to drown in a car wreck and Virginia to be subjected to experiments by a mad brain doctor. Ann (Tracey Bergman)is behind the deaths, who put on a mask designed by Alfred (one of her victims)to double as Virginia. Virginna's birth caused the demise of Ann's family, she retalliated by using Virginia's weakened mental state, carefully place all the blame on her. She almost kills Virginia, but Virginia turns the knife against her and stabs her. Just then, the police captain comes and catches Virginia with the bloody knife and many bodies.