Oh god, he hates you. Not as he would like, since whether he likes it or not, you continue to play the fraternal role in his messy life - but it's bad nonetheless. Sukuna hates that you're always there; he hates your healthy lungs when his should be charred. He hates your charming hair and ridiculously mesmerizing eyes. He hates how well you speak, how intelligent you are, how you can cook, how you're good at absolutely everything you do, how everyone at the university falls at your feet as if you were a shepherd preaching the gospel. He hates how your breath is always minty fresh, while his reeks of beer and whiskey. He hates the damn way your parents obviously prefer you. He hates it. He hates everything.
What the hell went so wrong? 'Look, Sukuna,' usually said when you're doing something amazing and the only thing he has are scars never truly healed from meaningless street fights. Looked crooked? That was already a reason. It's common for your parents to disapprove of him, he sort of gets it, but man, he has no one. Who fills his chest? Cheap hookers he casually buys? Cigarettes? The drink that burns his throat incisively? It wasn't that. Sukuna hates not being amazing. He truly hates not being like you. Yet the guy would rather be impaled until his organs come out through his mouth than admit, express it.
Once again, you did well.
University tests were inhumane - obviously, since, pufff, a renowned university, at least ten minimum wages as tuition. However, those damn tests always left him and practically everyone in the place crushed - but never you, apparently. Sukuna struggled just to get a '53%'? That's unfair. You're there again - coming home and confidently showing off the test with '100%' to your proud parents. Sukuna can't accept it. That's why he barges into your room after your parents' pride. "A test means nothing," he spat ironically, but with venom in his tongue. God, he just wants to be you.