WARNING SPOILERS AHEAD!
My view on what Kamui might have been thinking about after Subaru kills Seshiro. This my first X/1999 Fic so be gentel plaese.
Kamui just stood there and watched as the man who saved him from his own despair knelt on the pavement, holding the body of the man most precious to him, covered in his blood. Subaru had tears in his now mismatched eyes with his head down in mourning as he hugged Seishiro's body to his own. Subaru was quiet, but no doubt was cursing at the world in both his heart and mind.
He knew that Subaru was suffering, for he saw his own heart and despair reflected Subaru's when he helped him out of his own broken soul. Kamui knew that Subaru loved Seishiro, just like he loved Fuma. He saw himself in Subaru and Fuma in Seishiro. He is saddened to see this scene before him and cannot help but wonder, "Did it have to end this way?"
He watches as the barrier disappears and breaks just like the two men in front of him. Then he can't help but think, "Is this going to happen when I go against Fuma? Will he die by my hands, just as Seishiro died by the man he loved, Subaru? Or will he die by Fuma's hands? Is history going to repeat it's self only on the two of us next?"
His heart is slowly breaking at the thought for his wish is to save both Fuma and the present. He does, no, knows that he cannot kill the one he loves.
Kamui stood in Subaru's room later that night telling Subaru that they were the same in thought and listening about Subaru's thoughts of Seishiro avoidance of him. Subaru told him to focus on his wish so that it could come true. He knew his wish might cause misfortune for others, and was answered that, "There is no way for everyone to be happy." Subaru went into another catatonic state again and lost his will to make a barrier with the death of the only person he had left worth fighting for.
Even after hearing those words he still wanted to make his wish come true. Kamui would make his wish come true if it was the last thing he did, even if it meant that Fuma would realized all the death he had caused; even if it meant the end for himself.
So what did you think? R&R please.