What is this horror movie: son locked up in room for yrs escapes to murder houseguests in old B&W film?

I'm pretty sure it was a black and white movie and it was old when I saw it probably 20-ish years ago. I think that it took place at a mansion or inn with an eerie woman as the hostess. I believe her son had been locked away in his room for the majority of his life and escaped to start killing the guests. He had hugely overgrown finger nails that he used to slash car tires, trapping the guests and he poisoned one person who died with foam coming from the mouth and another was strangled with piano wire. I don't remember the rest, but it was really the only scarymovie I think I've ever seen!