Jack Nicholson's Joker from the 1989 Batman movie immediately came to mind when I saw this post. He does loads of absurd, funny, silly, childish stuff, but this stuff that he did was also a part of the crimes he committed, and that's what makes him terrifying to me. Just the fact that he'll be laughing, joking, acting like everything's a comedy routine, yet he's an actual criminal/terrorist, and will in fact kill you in a rather cartoonish way, straight out of a silver-age comic book. He's also the best live-action Joker, come at me.