bkdk is so ripe for "one fell first but the other fell harder" because its almost subtextual canon that izuku has loved katsuki his whole life, while katsuki takes time to discover it—but when he does, he fucking dies for him.
like izuku's is a long, sturdy, enduring love. a hard and fast constant that has been burning in the background of their relationship since they were fucking toddlers. he doesn't know how to do anything BUT love katsuki, it's so ingrained into him and completely bulletproof to anything that happens between them. katsuki is a little demon to him and still...still izuku loves him, and follows him, and waits for him. he's loved katsuki for so long, he probably doesn't know how to define the word without him.
but katsuki? katsuki probably disdained the concept. it felt weak, it was too sentimental, and he didn't exactly come from a home background that enabled him to stay in touch with his feelings. he has a hard time even understanding what they are, much less expressing them—so everything confusing gets turned into anger because that's the example he's known, and it comes easiest. it's the simplest to understand. reducing every little feeling down to the sheer rage he experiences over having them at all, his inferiority complex flaring up over what he percieves to be a vulnerability. it's why it rears its head so sharply around izuku—his bark grows more feral, his bite more harsh, because NOBODY makes him feel more vulnerable than weak, quirkless izuku.
it takes him a long time to figure out why. to untangle the knot of feelings he causes in him, to start unravelling and doing the mature work of identifying each thing, and learning how to express it as something less explosive, less harmful to the people around him.
so when he realizes that the reason his emotions grew so large and made him feel so much weaker around izuku weren't just for reasons of insecurity, but something else...it changes everything. that single moment of his first recognition is like a dam opening.
it pours into his chest and floods everything, soaking him to the bone with the feeling. it douses him so he stops sparking and snapping and booming into his fits of temper at the mere sight of izuku, because he realizes it was never anger he felt, it was love.
and now that he knows as much, he can't fight the creeping heat that climbs into his cheekbones. the rapid thudding in his chest. the open-mouthed awe of who izuku is, and what he's become, in spite of katsuki's hindrances to his progress. he's bowled over completely by the sheer depth of what izuku makes him feel, overwhelming him until he's smothered in the sensation.
once he knows what it is, he's too straightforward and no-nonsense to do anything other than face it head on and embrace it. there's no lying to himself, or pretending it's something different.
he loves izuku, and it's with every fiber of his being. it's with a force stronger than the collective output of his quirk, with a ferocity that burns hotter than the sun. it eats him alive and yet fills him with life at the same time.
in the course of a single day, he understands that he would do anything for him, would give anything. he understands that izuku matters more than anything else. his priorities shift, and it seems like it should be monumental, a veritable earthquake with the magnitude of the primary focus he's held since childhood changing, on a dime; but it isn't. it's just quiet acceptance. it's a calm settling, like fitting into place. like his realization has fixed something he hadn't even known was wrong, and now it was as it was always meant to be.
izuku is the most important thing in the world, and it's right, because izuku became his definition of being a hero, so chasing him really doesn't change anything much at all, does it?
and he knows. he knows izuku loves him, has always loved him, but he also...he also doesn't need him, not in the same way. sure, izuku is a little obsessive, and can be a little posessive, if pushed, but he's also grown from the meek, trembling, wisp of a boy he used to be. he has other people, friends that he can turn to. people who believe in him, support him, have never hurt him. and he's also developed into a ridiculously independent person as well—even to a self detrimental degree—as a result of the way katsuki pushed him away for so many years, ensuring he had no one to rely on but himself.
katsuki may still lie in the center of his universe, but he's no longer the only force of gravity. izuku has a whole solar system of people orbiting him, and he no longer needs katsuki the way he used to. and that's a good thing, a great thing—that he's overcome the isolation katsuki had selfishly inflicted on him, that he's becoming healthy and whole.
but katsuki feels like he's floundering. like maybe their roles are reversed, because looking at izuku smiling and laughing with people who aren't him feels like all the air is being sucked out of the room. because he's never really loved before—not like this, nothing like this—and suddenly it's everything. from the ends of his hair, through to his fingertips, down to the bottom of each and every nerve ending. he loves izuku, and it consumes him whole.
he loves izuku, and it means he could—he would die for him. he would sacrifice literally everything for him. he would do whatever it takes, to prove not just to izuku, but to himself that he's changed, that what he feels is the real thing, and that he's worthy of feeling it at all. he would give his life because maybe then it would chip into atoning for what he's done, the first step towards earning the love izuku has always so freely given. maybe it would justify having these feelings at all, when izuku is so far beyond his reach.
the thought no longer makes him angry. it no longer makes him feel small and infuriated over the idea that stupid, useless deku is standing over him. he knows izuku was never any of those things, and that he was always better than him. and now he's grown enough to respond as he always should have, by seeing that as the symbol of what he ought to be, what he needs to work towards, if he wants to be the kind of hero izuku seems to believe he can be. not for the goal of being number one, not to stand on top; but merely so that he's able to stand there beside izuku, worthy of standing shoulder to shoulder with him. he'll drag himself up, inch by gruelling inch if he has to—but he'll do anything just to be by his side. to fucking earn the privilege of it.
every single thing katsuki has ever defined himself or his life by shatters in an instant, like a mirror cracking and falling into so many shards, only to reconstruct and reflect back the image of izuku—his childhood friend, the love his life, the very meaning of the word hero.
so yes, izuku may have loved him first, and loved him longer, but katsuki's love is an earth-shattering, world-changing, redefining revelation, that entirely upends him and changes him. it breaks him down, tears him apart, and reconstructs him—so every last little piece is made up from the love he feels for izuku, so deep and so very essential to his very being that he suddenly finds there is no katsuki at all without him. he would not exist.
so when all is said and done, and the world is saved and katsuki didn't die after all, but izuku was forced to give up the one thing he's always wanted, the one thing he's worked so hard towards, the one thing he of all people was deserving of—well, katsuki doesn't even have to think about what to do next. doesn't have to contemplate for a single second whether or not he should devote his entire existence to correcting that wrong, to fixing the cosmic imbalance of justice.
he'd have died if it meant izuku got to keep his power, got to keep on being a hero. he gladly would have traded his life for that.
but he can't hang everything on what he thinks should have been—he'd be trapped forever on every little thing in the course of their lives he'd done wrong and wishes could have been different, if that were the case. he's smart enough to know that he has to move forward, has to make them better, has to grow and work and make them better, because that's all izuku has ever done, and he deserves the same effort in kind, ten times over.
every penny to his name, every spare minute (and even more that he can't spare, eating into his sleep hours and mealtimes,) is given to izuku. every ounce of effort, of his all-or-nothing, full steam ahead work ethic, every single bit of his drive and motivation and stubborn fixation on his goals, they're all izuku. it's all for him, for eight years straight. and it's nothing really—a drop in the pond, because what's eight years when katsuki wants the rest of forever by his side?
the rest of his life, hopelessly devoted to him, because katsuki doesn't know how to love any other way. and love him he does, wholly, helplessly, and utterly.