I think the solution to that is to just have put him in solitary and never let him out and don't toss the key because the cell will need to be used again.
There the guards are redundant. Just let nature take it's course.
An unpopular opinion (I have many) I'm sure. But think about who this horrible man was and the suffering of his victims.
I don't think human rights apply where the subject doesn't qualify as human.