In season 5 of Samurai Jack, Jack claims that he had previously never killed another human.
For the most part that appears true, but the creepy scientist in the Autumn season of death puzzled me. I know he looked quite strange, but he certainly was not a robot, and it is not immediately obvious that he wasn't human. The hyper stylized art style of Samurai Jack, whilst awesome, allowed for great variations of character design, and it was occasionally difficult to differentiate between aliens and humans (take the cursed blind archers for example).
Given that Jack killed the scientist, I am inclined to assume that he was indeed an alien, but is there any official word of whether or not that is the case?