Silent Night, Bloody Night (1972)
A man investigates unsolved murders in a former mental home he has inherited. Mary Woronov (DEATH RACE 2000) and John Carradine (THE UNEARTHLY, RED ZONE CUBA) star.
This film is public domain.
I think this movie is pretty damn creepy actually, but the Rifftrax boys would have a LOT of fun with it!