Sora limps his way toward Xehanort's defeated body as he attempts to writhe his broken body for one last attack at Sora, but he knows it's fruitless, all he can do is hold his arms up in defeat and beg for mercy for whatever precious few minutes of life left in his body. As Sora makes his way toward him, Sora reflects on how he has turned into a joke, some jester who's only move is to murder and destroy whatever is in his path, he has become exactly what the King and Yen Sid wanted him to be, some tool to uphold the never ending cycle of light and darkness, not that it matters anymore, they are all gone too.
Sora is about to finish Xehanort off when it becomes clear that his life has already slipped away, his last words are inaudible but obviously filled with anger and regret. Sora pauses a moment to reflect on how morality has seemed to disappear in the battle between light and darkness, the struggle becoming a ceaseless maw for death and destruction, he is unable to justify what he has done for no other reason than pure revenge, and that's fine.
The scene cuts to Sora on Destiny Islands, he has just finished making a grave to commemorate the fallen, he is alone, painfully so. The names of all his friends throughout the years litter the makeshift sign in the sand like reminders of all his failures. He apologizes to each name on the grave, for his weakness and inability to save them. As he arrives at Riku and Kairi's name he is unable to finish the apology, his pain is beyond words, and is punishment enough in his eyes. He realizes that the keyblade is not a tool to protect or to destroy, it is a tool to learn and to survive. And survive he did, for better or worse.
Sora then makes his way to the pier, the sun has almost set, he pauses to relive the flashback where Kairi makes him promise to never change. Sora brushes it off as yet another promise he couldn't keep, he can't be much more different than the kid who wanted to see the world, only to have the world come crashing down on him. The makeshift rafts Riku, Kairi, and he made are still bouncing off the end of the pier, as the waves gently move them back and forth.
Sora sits down on the edge of the pier, the sun nearly gone, for one reason or another, he closes his eyes and realizes that if he tried hard enough he could imagine Kairi next to him. She touches his hand, she tells him it's not his fault, and that it's all going to be all right. He wishes this moment could last forever, but in a flash, she's gone again.