I don’t think it’s that strange. Both Shisou and Loulan are among the main traitors to the empire that planned to overthrow it and attempted to kill royals. In an ancient empire, they weren’t going to be let alive. The royal guard likely intended to kill in the entire clan, which was also why Loulan poisoned the children to let them die a “peaceful” death (though, I wouldn’t be surprised if it was the resurrection potion).