Anyone who saw Tom Hanks in The Polar Express knows that cartoons are super funny, but living cartoons are just eerie. Slapstick was created when a high school kid who was looking for revenge on a classmate, accidentally fell into Dimension X (unconnected to the home dimension of TMNT villain Krang). When he crossed over, his body was turned into indestructible ectoplasm which is just a fancy way of saying Slapstick turned into a toon character from Who Framed Roger Rabbit. Which seems like a lot of fun until he snaps and starts beating the hell out of his allies.
Slapstick has always had a career as a hero, despite one telling incident that seems to have been forgotten simply because he's a cartoon. While he was part of The Initiative, he was trained by the small-timer hero Gauntlet. Unfortunately, Gauntlet decided to mouth off to Slapstick about his former buds in the New Warriors. So of course, Slapstick figured that the only possible course of action was to sneak up on Gauntlet when he was alone and beat him half to death using a hilarious cartoon mallet. It's a pretty brutal assault, but in the next panel, Slapstick laughing about it. This clown might not have the body count of the other giggling psychopaths on this list, but when you realize the insane scope of power a living cartoon would have, all in the hands of a looney tune that is prone to violent outbursts, he gets a lot less funny.
Sometimes, it's all in a name. For years, Maximus the Mad, brother to Inhuman leader and fellow alliteration enthusiast Black Bolt, has been hanging around the various Inhuman cities like that skeevy childless relative. Whether he's in jail, spouting cryptic threats, or sitting on the Inhuman throne after another supergenius scheme to get over on his big brother, Maximus just oozes sleaze..
This has never been more true than in his dealings with Black Bolt's (most of the time) wife, Medusa. At various times, Maximus has turned the object of his obsession into his queen or dosed her with psychedelic drugs. But the pinnacle of his aggressive awfulness was probably in the late-90s Inhumans mini-series, in which Maximus spends a few days licking Medusa's face before cutting off all of her living hair. And Medusa's living hair is kind of her thing. To her, a haircut of this magnitude amounts to genital mutilation.
Of course this is to say nothing of the times Maximus played Dr. Mengele with the troglodyte-like Alpha Primitives, or the time he assembled a guilt-powered robot. All yet he still has the gall to say that all he ever wanted was for his family to like him.
Madcap has been abusing his body for fun and crime since the day he was reborn. After a classic comic book car crash involving experimental chemicals, Madcap gained the powers of instant regeneration. The only downside was the part where the crash killed his entire family and drove him instantly insane. In an extremely un-comic-book villain move, when he got out of the hospital, instead of taking out his new insanity on the heroes of the world, Madcap straight up tried to kill himself. Thanks to his newfound curse of instant regeneration, he lived. It was only then Madcap turned to crime.
As a bonus, Madcap can also turn people insane just by staring at them. Just 'cause.
At one point, Madcap was completely destroyed along with Deadpool, surviving as another voice in the Merc With A Mouth's head when they both regenerated. The Joker might be claiming to be immortal these days, but Madcap is the only psychopath who can actually claim the title.