Mental health care sucks, even when in the system it still sucks. Yes thats a terrible thing, it should be addressed.
In this case its a little to late
I find rushing to the perpetrators aid to be frankly kind of wrong.
So yes i think you are wrong for immediately going "Oh the poor guy who stabbed that girl, maybe he had mental problems" honestly after a point i do not care, if he is mentally disturbed enough that he dos that, then the effective consequences should be the same. He is put in a place, a door is locked and he is kept inside there because he is dangerous. Prisons and asylums work on the same principle.
There is a world of difference between somebody who has mental problems and responsibly handles them and somebody who dos not, peaple who have severe enough delusions to have "space fairies" tell them to kill people have probably been diagnosed if thats the case he should be on medication. If he is not that is likely to be a result of his own actions. So no my sympathy dos not rise very high.