Seitan Taisei certainly looks malicious, and he's destructive to everything around him, but I see him as the destructive side of nature, like Goku is the constructive part: the hurricane vs. the necessary monsoon. Seitan Taisei doesn't speak, he can't be reasoned with, and his malice seems more animal than human, the way that a cat plays with a mouse and clearly enjoys the game, but isn't sadistic in the human sense. I don't think Seitan Taisei knows the difference between right and wrong, which makes his killing sprees different from Gonou's.