I am sure no one would disagree that Khaun takes the cake, but whom else would be considered cruel? Perhaps the Klingon from Day of the Dove? Perhaps Lord Garth? Perhaps General Greene from a season 3 episode? General Greene pretended to be friendly with Kirk but tricked him, only to have murdered a Vulcan and Abraham Lincoln as well! In history Greene came before Lincoln, but what a cruel character no doubt whom wants victory and advancement at any cost. After I finish season 3 I am gonna re-watch various episodes in the first 2 seasons to become more familiar with them. But there is a Klingon episode from season 1 that featured a cruel Klingon whom may be considered. Whom else?