I think the opening question is disgusting. Mentally ill people are more likely to be the victims of crime than the perpetrators of it.
So called normal people carry out atrocities on a daily basis. They plan terrorist acts and plant bombs. They rob banks and murder people.
It's easy to say a person who guns down multiple victims must be a schizophrenic or a person with psychological issues because that's how TV programmes have presented them for years (criminal minds etc) but the facts present a much different story.