That’s a nice running theme with a lot of the Marine characters, they each have an ideal of what justice is or should be but they inevitably clash with reality. Garp loses faith in the justice preached by the Marines after the God Valley Incident but resolves to stay and protect the low rankers from what they can’t stand up against, only to find he can’t protect everyone when the WG is set to execute Ace. Ultimately he recognizes the limits of his personal justice which is why he’s training the next generation vs Sengoku who’s basically done with trying to change things after retiring.