The thought of sleeping without sopor slime scares Karkat shitless.
In theory, the purpose of the slime is to calm the racial inclination for violence and carnage in a young troll's mind enough for the child to develop into a reasonably intelligent individual that can be of use to society. Most troll children appreciate it because the dreams they have otherwise are overwhelming in their sheer bloodthirst and savagery. They can take it once in a while, and more often as they grow up and grow used to sifting through their own dark subconscious. As adults, many trolls stop using the slime altogether, preferring to channel the bloodthirst in ways productive to the Empire and their own ambitions.
Karkat is schoolfed enough to understand how it works. He's also perfectly bloodthirsty and savage as it is, and he doesn't doubt that if that was all there was to it, dreaming about cutting a bloody streak through hordes of screaming grubs or whatever would be nothing but a pleasure. An asshole rumpus romp in a sea of blood, who cares.
He would tell no one this, but he knows his dreams are different than everyone else's. It's probably because of his mutant blood - one of the perks of being a freak is getting to be that much more fucked up than any other fucked up troll. And it scares him shithive maggots.
Karkat doesn't particularly like sleeping even in his recuperacoon. The slime helps, it's not that it doesn't, but even so, he often wakes up with an unshakable queasy feeling of wrongness in his guts. He would never mention that to anyone either. He'd tell them sleep is a fucking waste of time. And despite everything, he will make sure never to tire himself out so much that he'd fall asleep without getting to the sopor slime. He knows what will happen if he neglects it.
Sweeps ago, when he was even younger and even stupider than now, he took a nap on the floor. Only once, but that was enough. He was tired from sparring with his lusus, but it was far from bedtime, and he just wanted to rest for a while. (In other words, he wasn't just young and stupid, he was weak.) He curled up next to the crab beast and closed his eyes. He knew he might have dreams, but he didn't care at the time. Perhaps he was even curious.
Despite sweeps of attempting to suppress that one dream he can still remember it clearly, as bright and painful as the Alternian sun.
In the dream, Karkat is no warrior. He's not fighting, he's not killing, he's not destroying anything. The only blood that is being spilled is his own, brilliant red rivers trickling from the wounds all over his broken body. At first he can't even feel it, only stare at the color that he'd fought so hard to conceal being revealed all over. This is wrong, he thinks. But what's wrong isn't the color of his blood, or the fact that he's bleeding in the first place -- it goes a lot further than that. Everything is wrong. The world is wrong. We have to put it right, but it is too late now.
There are people watching him, adult trolls with familiar and unfamiliar faces. Some look at him with pity, some with hatred, most with cold disdain. There's something wrong with all of them, and it breaks his heart. He wants to save them, but he knows he can't.
It is only when he's settled into the dream, guts churning with the wrongness of it all, that he starts to feel his own body. His wrists, suspended above his head, are burning in agony, clamped into red-hot irons. His body is bruised and bleeding, skin ripped with dull instruments exposing flesh beneath, bones broken, inner organs pierced with barbed needles. Every breath and every beat of his heart brings new agony, and it doesn't feel like a dream at all -- it's real, it hurts like nothing is ever supposed to hurt, and he knows he's being tortured to death.
And that isn't even the worst part. The worst part is the rage. Karkat would never even try to explain this to his friends, because anyone who knows him at all knows that raging is one of the things he does best. But it's different in the dream. It's a blazing, powerful, crushing rage that should make him feel empowered, but doesn't. It's not directed at his tormentors. It's not directed at the world for being one huge fucking mistake. It's solely directed at the one who made that mistake, inwards, breaking his soul as thoroughly as the torturers break his body. He knows, beyond any shadow of a doubt, that no one but him could ever inspire or deserve such rage. He draws a ragged breath and curses his heart out, and with that he wakes up.
He remembers that he woke up screaming. His lusus must have been startled, but Karkat barely noticed, his whole body tense and aching, red-tinted tears flowing unstoppable from his eyes. He spent the rest of that day screaming and beating anything with reach of hands and feet, walls and furniture and lusus alike, except when he was curled up in a corner, sobbing uncontrollably.
He got over it. Sort of. It was just a goddamn dream. But the very thought of going through that again practically makes him piss his pants. Just no, no way no how.
When his recuperacoon breaks during a kerfuffle in his house some time into the game, with no easily available way to have it replaced and refilled, Karkat vows to never sleep again.