Love is a powerful emotion, but nothing holds power in the face of destiny.
Fuuma knows he is Kamui's "Twin Star." Furthermore, he knows what that entails, and he bows to his destiny; when you're destroying your own heart, the fate of humanity is meaningless. Fuuma doesn't exactly want to end the world, but destiny will prevail.
Fuuma's always know that destiny was going to run his life, ruin his life. His mother died, because it was preordained. His best friend left, because it was preordained. Kamui fought his destiny, but Fuuma has never tried. He never even realized he could, until after.
After Kamui chose a destiny, giving Fuuma the opposite. After Kamui became "the Kamui," and Fuuma was made "the dark Kamui." Fuuma, in that instant, wasn't fighting destiny. Why, if destiny could be fought, didn't he know that? Why hadn't Kamui shared this knowledge? Why had Kamui betrayed him like that? For Fuuma, destiny is a cage; and somehow, Kamui holds the key.
Fuuma hates Kamui, for not teaching him to fight destiny, and so destiny, once more, wins. Fuuma is Kamui's twin star.
Fumma is like a billion other people.
Fuuma wishes that destiny were different.