People too stupid to know right from wrong should never be allowed to walk free in society.

The real crime here is that he won't be locked up for life. He will harm someone when he gets out and the people letting him go should be held accountable for his actions once he is released.

Being proven too mentally incompetent to stand trial should be immediate life in some sort of institution. Preferably where they are required to do simple, repetitive tasks to help defray some of the cost of segregating them from the rest of society.