This is the consequence of having any person like the two highlighted in this article living and breathing and having the ability to interact with any other human. If you are going to lock them up and never let them interact with anyone ever again, then that is considered torture as well: https://www.washingtonpost.com/opinions/george-will-the-tort...
So the choice is to either let them kill each other (you can try to stop it, but so long as they have physical access to other people they will kill them for no reason), or you don't allow them that access and this is torturing them. Seems like a Catch-22.
For my money putting horrible people like this together is a way of letting them solve societies problems by killing each other. It's certainly not desirable to have this happen, but clearly nobody is solving it in any other way.