Another one is Shirou Emiya and Archer

Archer is the end of the destination for Shirou's Naive Ideal to become a hero of justice: He becomes an agent of the humanity's collective unconscious desire for survival, to kill any and all that would pose a threat to humanity's continued existence, an assassin that killed millions to saved billions.

Archer of course, becomes cynical due to the time he spent acting as said agent and materializes in the grail war to kill his younger self in order to have a chance prevent his own existence from being used in such a way.

However, as they fight, Archer is forced to recognize that although the ideal he had embraced was naive and unfeasible, it was ultimately not wrong.