don't look in the basement (1973)

aka The Forgotten. Death Ward #13. Don't Go in the Basement. The Snake Pit.
"We are a family, and everyone helps with the chores."
When the Dr running an insane asylum is killed by one of its inmates, Dr Master's (Annabelle Weenick) decides to take over the hospital, as if nothing has happened. Then Charlotte Beale (Rosie Holotik), a new nurse arrives unexpectedly, and gets a less than cordial welcome from both the Dr and the inmates.
Despite it's cheap look, lack of acting talent, and slow pacing, if you can keep watching, you will be rewarded with the most obvious of twists.