Love is a powerful emotion, and guilt about loving is more powerful yet.
Kamui loves Fuuma, still, despite all that Fuuma has done. Kamui still wants to save Fuuma, to protect him.
Still wants to be loved by him.
Kamui feels guilty about loving somebody who has killed so many people, feels guilt towards the people who are dead.
Kamui feels guilty, as well, for destroying Fuuma's life. It was his choice, his love, that made Fuuma a monster. Kamui almost wishes he'd never loved Fuuma.
Kamui loves Subaru, too, and feels guilty about dividing his love between two people. Fuuma, he loves with the weight of all of his life, years spent protecting and being protected. Fuuma, he has always loved. Subaru, he loves as a teacher, a mentor, and a kindred spirit. Kamui loves Subaru because Subaru so desperately needs to be loved.
Kamui loves Subaru because he needs to love somebody who won't be hurt by that love. He feels guilty about using Subaru as that person, but what does one sin matter after all that guilt? Besides, Subaru doesn't mind.
Kamui wishes he'd loved Subaru sooner, better, longer, because then Fuuma might still be with him.