Riku asks himself if it's really worth it more often than he'd care to admit. He reminds himself constantly that this life in the shadows is his atonement for what he's done – he chose to ally himself with darkness and now he has to pay the price. But he isn't noble enough to accept his sentence without railing against everything he's been asked to sacrifice and the sacrifices he knows are yet to be made.
It starts with letting himself be sealed in Kingdom Hearts, giving up his chance to go home for a life of exile and uncertainty. Mickey and Ansem don't show any leniency toward him, pushing him harder and harder to achieve their ends. First his eyes are taken, their bright sea green turning to the unmistakable yellow of the Heartless. But even then he can't defeat Roxas, this shadow of Sora who wields the dual Keyblades with deadly skill and leaves him gasping on the ground. He knows then that he has to delve deeper into the darkness he both fears and despises, sacrificing Riku to become Xehanort. He covers all the mirrors in their hideout because he can longer bear to see himself.
Sora wakes up and even though Riku wants to explain everything to him he doesn't think he has the strength to see the disgust and hatred on his friend's face when he looks at him and sees only the man who ruined their childhood innocence. So he remains in the shadows hidden beneath the cloak of Organization XIII so that the body of this abhorrent form cannot be seen and wonders if, in the end, it will all be worth it.