also the fact that his remains were shipped to an army camp, then he was posthumously beheaded and his skull was shipped to the Smithsonian instead of his family, where it stayed for 72 years before his descendants became aware it was there and appealed to have it returned, and then then it took another 14 years before his skull was actually returned to them, is so incredibly far beyond fucked.
According to the article, it looks like the context in which he killed the general was in an ambush during negotiations. I figure that that is why it was considerer a war crime.