You are an intelligent person. You HAVE to know that I do not mean someone with autism or a developmental disorder. That would be sick and repugnant. But that's not what I mean. And you know it.
What I mean is someone with such a mental illness or sickness or reality distortion that they can justify picking up a gun and killing twenty children. A person has to be sick in the head to do something like that. Sick in the heart too. Use whatever terms you care to pull out of the dictionary but we all know what we are talking about.
Someone has to be "crazy" (define it as you wish) to behave in such ways.