The Diner WAS true, sort of

I read a similar story in a book called, World's Greatest Ghosts. Here's how the real story went:

In 1922, a couple in Hungary, who ran a bed and breakfast, were killing people out of greed, not mercy.

One day, a very well dressed man was a guest and they started getting to know him. He carried large sums of cash on him, and they wanted it. Later on, they poisoned him. In his suitcase, they found a picture of him and his parents. They recognized the picture as themselves with their son they'd driven away from home years earlier for his bad grades at school. They'd also driven away their daughter, who later became a prostitute! Horrified that they'd killed their own son, they poisoned themselves.

I think this is what happened to the diner owners, but the show went off before we could see it.