Kirei Kotomine in Fate Zero is one of the most terrifying characters there is, what makes Kirei so scary can be said as the same qualities that made a man like Heydrich so terrifying. With his lack of feeling or attachment, Kirei did not care that he killed, in fact, he did not think he was crazy because he killed people but rather because he had no interest, nothing he genuinely liked. It was the fact he was a man with no attachments searching for himself, that made Kiritsugu so deathly afraid of Kirei. In fact, Kirei after he embraced his sadism was, to me, less terrifying compared to Kirei before that. Kirei prior to embracing his sadism had no limits or qualms about anything, nothing he enjoyed, and thus nothing he had to follow. Post sadist Kirei would at least emotionally torture you before killing you but with Zero Kirei, you never know what you're going to get. Thus Kirei Kotomine is scary not because he enjoyed harming others but because he enjoyed nothing.
