People have always killed, including often non-combatants, due to differences in religion, skin colour, nationality, tribe, etc.  USA dropped atomic bombs on civilians to end the war with the Japanese people, the Allies bombed the civilian population of Dresden, just to give two examples.  You would have to expand your definition of "psychopath" beyond ISIS if it applies to people who kill others because they belong to another group.  (P.S. It's "checkered".)