I do however wish it had been more "I need to put things right" and less "I need to be punished for what I did". ›››
Considering that in real life (as opposed to the chart morals of D&D) there is no clear division between "right" and "wrong", and considering The Nameless One lived lives where he was lawful, chaotic, good and evil, and any combination of all those, I interpreted the ending of the game as a form of finally accepting your destiny and facing the consequences of your actions.
The main reason why TNO became immortal was to evade the eternity of servitude in the Blood Wars as a consequence of his evil acts. Having achieved immortality at the cost of part of his humanity, he lost his sense of purpose by losing the memories of his former life and forgetting the very reason of his current state.
He must have been exhausted from wandering the city of Sigil and possibly other planes (he had been in the fortress of regrets before) for countless years. When he finally regains his memories, meets his former selves, and overpowers the Transcendent One, a form of closure is submitting to his original fate that was interrupted when Ravel split him.
He lived multiple lives of torment and brought torment to those around him (enslaving, deceiving, etc.). At this point, even Baator or the Abyss are a relief, or at least a change.
I usually don't play goody-two-shoes characters, so that's my interpretation of the ending
All I'm saying is what we see is very open ended, and doesn't necessarily have to be one thing or another.
Facio, Voco, Ferre.
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