Psycho 2
(1983)

Mary Loomis (Meg Tilly) kills Dr. Raymond (Robert Loggia) thinking he was Norman Bates (Anthony Perkins).  As she is about to fatally stab Norman, the Sheriff shoots and kills her.  At the station, the Sheriff theorizes that Mary and Lila Loomis (Vera Miles reprising her role) were the murderers of Mr. Toomey (Dennis Franz) and the teenage boy and that Mary even killed Lila, her own mother, in an attempt to make Norman fall into his psychotic ways and be taken back into the custody of the state.

Mrs. Spool (Claudia Bryar) is revealed as the real mother of Norman Bates and the true murderer of Toomey, the teenage boy and Lila Loomis.  Her sister raised baby Norman as her own child after Mrs. Spool was sent to a mental facility.  However, Norman poisons Mrs. Spool's tea and bashes her head with a shovel, then sets her at the rightful place in the window.

Thanks John K!