Sometimes people just forget what cats were originally domesticated for.
Rodent extermination. That's why we have my cats, cats show up here every now and again and we feed them, take care of them, etc, but they decide if they want to live or die.
If they decide they stay inside the walls at night, they live, if not, then they die. It's simple. Keeping them inside is probably more cruel then letting them be free.
Allowing them to do their job isn't a bad thing.