If killing people because you do not like them is a mental disease then that is the context from which America emerged. It literally, articulated and prosecuted a genocidal movement to kill as many Red people it could and wipe away the cultures of the remainder.
We do not even know how many died...the estimates are anywhere from 8 to 80 million!. And then there were the over 2 million slaves brought in. America was never a record of mans charity to man. It was never about one people before god. It was a hierarchical structure with white men at the top and all others below and without political voice.
America became and is becoming that shining light on the hill. Trump is a rock in the path of that ideal. We should not be arguing about who is mentally ill but begin a search for a holistic approach to guns. This kind of ransom killing is a disease unique to this country among modern democracies. It is therefore not a disease but a cultivated event nurtured by a whole spectrum of things from racism to easy access to guns to the NRA and Trump. All must be looked at