Sora's breath shook and his knees fell; his hands jumped to muffle his sobs as he finally broke. Donald and Goofy were gone; gone like they were when Riku took the keyblade; gone like they were before he'd met them, wandering around Traverse Town without a clue in the world, chased by those monsters. Another sob broke through as he remembered his broken promise; a broken promise to a wayward friend, forever forgotten in the abyss of darkness and chaos.
"Take care of her," parting words for a parting soul, sealed for eternity behind that door; sealed inwardly and outwardly—no chance for escape, for heartless or for human. He'd closed the door right in Riku's face, condemning him to whatever terrors lie behind the safety of the light. Closed it without a second thought and without premonition; closed it without any hesitation and with no pure signs of remorse.
He truly was a heartless; he didn't find mercy for his longtime friend as he sealed the door and ran after the redhead he loved. He'd broken his promise the second he let her hand leave his. He abandoned her—he didn't know if he would ever come back, as he had promised, and promptly broken, to her.
He told Riku that he'd take care of her and he ran off on another adventure not to watch after Kairi, but to save Riku. Sora suddenly felt so horrible; he felt as though he was running amuck without a heart, as though his heart hadn't truly been returned to him when he came back from his heartless form, as though his heart hadn't been replenished when Roxas gave his life to him.
His own proclamations rang mockingly through his head; claims that the heart would never give in and never give up and that there was always a light that was never quite snuffed. He'd seen that light disappear too many times.
Cloud's was gone as he fought to control the darkness surging through him by the hands of Sephiroth and Riku's was gone as he let himself be sealed behind the door. Axel's light was snuffed out completely as he sacrificed himself to give Sora life; Axel was Roxas, a martyr just the same. So many worlds had been destroyed at the hands of the heartless and Xehanort's heartless, vanquished and never to return—those worlds that Sora thought he had restored but never saw, even as his second journey took place.
The nobodies had light. Sora was sure of that now, as Roxas mocked him and told him how wrong he was to do such things. He'd snuffed their light right out; he'd killed so many and thought nothing of it. He never stopped to think of who the heartless and these nobodies derived from. These people, who may have had a chance of being restored if their hearts could connect their nobodies and heartless together, would never be alive again thanks to Sora. He killed their chance, unintentionally, all the same.
He didn't even want to begin to think of what harm he had caused to Donald and Goofy, his comrades and partners. He'd fought alongside them for two years, vanquishing heartless and nobodies alike, but at what cost for the short tempered duck and silly dog? They weren't reunited with their king until the end of their long journey. Sora had even sealed King Mickey behind that damned door along with Riku.
It wasn't right. Nothing Sora did was properly justified; he did what he did for his own selfish reasons in the end.
He did what he did to find the girl he loved and find his best friend.
He didn't do it to save the worlds, and he didn't do it to help others.
That just came with the job.