What about the other one who was simply jailed for robbery? Did that deserve a death sentence? Or the one killed in 2015?
In general, I believe we have a moral obligation both to lock dangerous criminals up for an extremely long time to protect the innocent and to keep those criminals safe once they are locked up with a limited ability to protect themselves. In the case of predators who harm children, and when the courts also fail (this predator was ruled incompetent to stand trial), my feelings become ambivalent.
I'm not saying people like this killer should knock off everyone undesirable behind bars - I don't think that should happen. I am saying it would surprise me if locking the two of them up together was an accident. I assume someone in authority wanted this to happen.