Put the Laughter In Slaughter

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 * Accelerator of A Certain Magical Index laughs insanely during every single one of his fights, most of which involve lots of killing.
 * Higurashi no Naku Koro ni: There are pictures of her face with "You can't spell slaughter without laughter" captioned to them around on the internet, and she's pretty much poster child for this trope.
 * The Gundam franchise's three big Psychos For Hire, Yazan Gable, Ali Al-Saachez, and Decil Galette are all guilty of this, chuckling to themselves as they gun down MS after MS. Gundam Seed and Gundam Seed Destiny's Extended are not better about this, nor are their creators, Muruta Azrael and Lord Djibril.
 * Jinnai from El-Hazard: The Magnificent World is prone to wild laughter at any time he imagines the destruction of his enemies, but especially so on the battlefield where he directs an entire army of giant sentient insects against the human nations of El-Hazard.

Comic Books

 * The Joker in most of his incarnations, except when he's taking killing seriously.
 * As demonstrated in the former page picture, in The Dark Knight, he actually rides a circus truck emblazoned with "laughter is the best medicine", and a large red S spray-painted just before "laughter".
 * Actually, in his very first appearance, he didn't laugh or smile at all. Needless to say, Characterization Marches On.
 * Spider-Man's foe Carnage also takes great joy in killing, as his name oh-so-subtley hints at.
 * Bullseye is this to a T. His sometimes friend/sometimes enemy Deadpool is also a good example.

Fan Works

 * Pinkie Pie in the infamous My Little Pony: Friendship Is Magic fanfic Cupcakes, who giggles at doing horrible things to her victims.

Film

 * Any action movie that is filled enough with Narm has this, from the viewer's POV.
 * Hot Shots Part Deux is the most violent movie ever made! And it has a lot of fun proving that.
 * Jack Nicholson at the climax of The Shining was partly this.
 * There's The Toblerone, too!!! Hahaha!!!
 * Christopher Walken's Corrupt Corporate Executive in A View to a Kill likes gunning down his own henchmen way, way too much.
 * In Chris Nolan's The Dark Knight, Joker spray paints the extra "S" on the circus van that has the slogan "Laughter is the best medicine" to invoke this trope.

Literature

 * Bellatrix Lestrange from Harry Potter. An authentic Ax Crazy if she had an ax instead of a wand.
 * Odiana in the Codex Alera is completely Ax Crazy and really quite cheerful (and self-aware) about it. She usually expresses this by being inordinately cute and cheery while suggesting that they just kill everyone in about a 5-mile radius.
 * Rachel from Animorphs is a rare heroic (well, sort of) example of this. Right before charging into  she leaves us with this little gem: "I felt...I felt exalted. It was my moment. This was my place and my time and my own perfection. I was no longer afraid. Weird. If I'd had a mouth I'd have smiled.  I said. No one moved.  I asked. No answer.  I said, almost laughing. I lunged..."
 * Tulkas, the god of war in J. R. R. Tolkien's universe, is a rare good example of this.

Live Action TV

 * In Heroes, Sylar has a good bit of fun being a psychotic serial killer.
 * The easiest way to describe the new series Doctor Who version of the Master is "completely insane...and loving every second of it."
 * River Song is a Sociopathic Hero version, as she takes great delight in shooting down the bad aliens.

Tabletop Games

 * The Yozi Adorjan is infamous for this; the only sound she can stand is her own laughter, she swoops around the Demon City slaying all she touches, and murder is something she finds particularly amusing. Those who take enough of her Charms will also eventually adopt this attitude.

Video Games

 * Any FPS that is Duke Nukem-ish enough has this. We should open a Troper Tales page for all the times we as players have started laughing uncontrollably in the middle of a slaughterfest.
 * The characters in Left 4 Dead have a cackling sound file that some players set to play whenever they fired their guns.
 * Caleb from Blood also periodically laughs maniacally when there's lots of violence happening on the screen... Such as when he just threw a bundle of dynamite into a crowd of evil cultists (and/or innocent civilians).
 * during her in Higurashi no Naku Koro ni.
 * at a bare minimum during her murder of, in Umineko no Naku Koro ni. So much so that once just wasn't enough. And actually, Beato herself quite a few times, although.
 * Luca Blight. His motto might as well be "You can't spell 'Slaughter' without 'Laughter'!"
 * Kefka, people! Just one of the many things he has in common with The Joker.
 * In Team Fortress 2, the Heavy Weapons Guy is notable for this, especially towards the end of the "Meet the Heavy" video. The other characters, too, apply when dominating an opponent.
 * Wallachia of Melty Blood laughs madly even as he chops his opponents to bits. Not that you see any blood, but that's the impression.
 * Dimentio in Super Paper Mario does this after
 * Dawn of War: Khorne Berserkers, Chaos Space Marines and Ork Big Meks all have laughing sounds when attacking. Aspiring Champions even have a fast-paced laugh when their morale is gone.
 * In Borderlands, Brick, the Berserker, seems to have a lot of fun beating people into a pulp with his bare hands while in his Unstoppable Rage.
 * In |Sonic the Hedgehog 2006, Mephiles laughs like a lunatic after

Web Original

 * Flippy from Happy Tree Friends when he goes Ax Crazy.

Western Animation
"Popeye: You're not a riff-raff... Brutus: If ya fights and ya laugh... Popeye: With Popeye the Sailor Man! (toot toot!)"
 * In his last scene, Hank Scorpio in The Simpsons grabs a flamethrower and jumps down to battle a bunch of soldiers invading his secret lair. He laughs maniacally as they all desperately run for cover, and while Homer is leaving you can hear Scorpio shouting to him that he's "missing out on some fun!"
 * One episode of Drawn Together features a sickeningly-sweet Strawberry Shortcake clone who's actually committing a Holocaust. But, she reminds us, "You can't spell slaughter without laughter!"
 * The 1961 Popeye cartoon "It Hurts Only When They Laugh" has Olive making Popeye and Brutus laugh while she's doing the dishes so she'll know they're not misbehaving and fighting. However, while Olive is doing the dishes, Popeye and Brutus are beating the living hell out of each other—and destroying the living room in the process—while laughing. The boys lampshaded it at the end in Popeye's ending theme tag: