With no offense to those who believe differently, I do not care if someone is mentally ill or otherwise troubled If they kill/harm an innocent person. I am sad/sorry for the mentally ill, I sympathise with them and have worked in educational functions with many. BUT like with any other category of person: If they harm/kill innocents all my care and sympathy is with the innocent person. I have none for what did the harm/killing. None at all. I even have to fight reacting when their families/relatives support them over it and do not even begin to understand when the families of the harmed/destroyed say they understand and support the not victim.
I wish execution etc. worked as a deterrant for others, but I know the murderer (etc) who is executed will not ever harm/kill again. By the by, I do have a scale by which I would cover the straight, fast, not expected execution and the other things I advocate. The fictional Lenny would fit it (and I like the ending of that story), but this guy doesn't - he has too much data in him to qualify.