Gamzee (diamonds) Karkat: AU where Karkat is the one who goes on a killing spree
The horror-terrors were in his dreams, when he allowed himself to sleep. They whispered dark things in his mind, trapped him the darkness until he screamed himself awake. He tried not to sleep, but kept slipping back into their eldritch embrace when he couldn't keep his eyes open any more. Their abominable shrieks were demanding and he didn't understand them. Sometimes he even heard them while he was awake.
He just knew they wanted something from him, and that if they drove him mad with their whispers, he would do it (whatever it was). He started isolating, if only to keep the others from listening to him screaming in his sleep. He was stupidly angry all the time; his frayed nerves only compounded the problem.
When he rejoined the group, his screaming matches with Eridan and Terezi were monumental. He couldn't stand Terezi, his mixed feelings for her roiled inside him until he couldn't remember why he'd been flushed for her anymore. All he knew was that she pushed the part of him that was already listening to the menacing whispers into the light. And Eridan was defending her, when he tried pushing her away.
He hated them both.
One night, when he awakened from his nightly forced audience with the Old Gods, his mind snapped. Too long without proper sleep, and emotions manipulated by the cruel voices that whispered in his ears even when he was waking. His rage was red hot, and he grabbed his sickle and went to join the others.
Except the control room was empty of everyone but Eridan, Feferi, and Sollux. Things were getting heated between the three, and Karkat's arrival put a halt to a brewing fight, but not before Eridan managed to shoot Feferi through the stomach. And then he sliced Eridan's throat open from behind him, buried his sickle in Sollux's navel and sliced upwards.
Dark energy radiated off him, and he still only felt rage. There was still more to do. He let loose a primal shriek, and set off again. The tendrils that radiated from his black shroud licked at the edges of the meteor's halls, and he sensed the life of his enemies (friends?) through them.
He found Tavros' body next. He felt nothing, nudged the corpse with his toe, and moved on. Vriska he found next, beaten and having bled out on the steps. Again, he felt nothing. He stepped over her, and continued climbing. Nepeta confronted him before he reached the top, with claws bared and olive tears glistening on her face. He disemboweled her and moved on.
Equius showed up later, nursing bruises and near out of his mind with grief. Karkat's grim shroud put him off balance, as it was probably one of the last things he expected. Equius was in the way however, so swift kick managed to push the blue blood off the stairs and down into the void below. And Karkat continued to ascend the steps. The dark part of him that was enthralled with this new power and speed wondered where Terezi was, wondered if she would be afraid.
If she would scream, when he killed her.
Karkat made it to the rooftop with his sickle having gone sticky with multi-colored blood, and basked in the attention that was turned to him. Terezi stared in his direction, brow furrowed. He must smell different, cloaked as he was in the horror-terrors' powers. Gamzee was there, colored with cerulean blood and in a defensive pose, but he looked up at Karkat with a sort of sadness. And Kanaya as well, but she just looked worried.
He smiled at them all, and the screams of his masters bid him forward. Sickle raised, he started towards them at a pace that quickly became a run. He was screaming, and maybe there were tears as well, and he prepared to cut Kanaya in half with one fell swing. However, Gamzee stepped between the sickle and the jade blood at the last second.
"Shh, brother," Gamzee soothed, allowing the weapon to cut into his shoulder. Putting his big hands on Karkat's face, Gamzee soothed him. "Shh, it's okay." The viciously writhing black aura calmed, and Karkat slouched into the touch. "Shh."
Karkat purred, and Gamzee twisted his hands suddenly. The mutant fell to the rooftop, neck snapped. And the clown turned back to Terezi and Kanaya with eyes that flashed red.
"Welcome, to the Dark Circus."