A lot of people have this issue where they feel if someone does something "bad", they deserve something "bad" in response.
They ignore context and the situation. If the person was mentally ill, and that's why they did what they did, how is it "just" that they be injured or killed?
It's quite the assumption that this individual was mentally ill. We don't know anything yet.
I find it quite amazing that people are concerned over the suspects injury, on a hunch that they must be mentally ill.
Considering he's just chopped someone's head off, their injury is the least of my concern.