He is in love with him.
He does not know how it happened, when it happened, only that it did.
He knows his feelings are not completely one-sided. In those moments when they are alone together, when their eyes lock for a breath too long, he is not oblivious to the mix of affection, longing and hungry desire that he sees.
It still hurts though. He is in love, but it hurts. It fills him with red-hot rage but not the kind familiar to him. It differs from the typical explosive anger that colours what sometimes feels like his entire personality.
No, this is internal seething anger. The kind borne from a volatile mix of frustration and inferiority. The kind he hides to the best of his ability, lest anybody realise what he is so angry about. This is the type of anger that coils painfully in his belly and forces him to constantly tense his muscles so he does not shake apart because of it.
Katsuki is in love with Kirishima Eijiro, but he knows, knows he can't give him what he needs.
It does not matter how hard he trains or how much he wishes. Nothing will change the fact that he simply does not measure up. And that is what kills him.