B&W: Son Kept Hidden in Mansion Slashes Guests' Tires With Overgrown Fingernails

by Ali
(Port Orchard, Washington, USA)

Thanks in advance for your help remembering this black & white film I saw bits and pieces of as a child.

An eerie woman that lives in a mansion has several house guests that are trapped there (maybe due to a storm?) and they begin to be killed off, one or two at a time.

One person is poisoned and foam comes from their mouth, and another person is strangled with a piano wire. It turns out the killer is the eerie woman's son that she'd locked up in his room for years.

He had enormously overgrown fingernails, which he used to slash car tires so no one could leave. He made his way unseen through the house through secret passages.

Ring a bell?