I think probably for him he can do more good being a marine than being a pirate? Most pirates are actually bad, also why he doesn't want to be an admiral to not be an actual lapdog to the Elders and Celestial Dragons.
Sure, but on a counterpoint he could just be a good pirate like Luffy lol there's no rules to being a criminal, yet he is obligated to defend evil as a marine
Well he doesn't want to be a pirate lol, he wants to be a marine. Garp never wanted to be a pirate because his identity and pride were tied to being a marine. His goal was always to uphold his own sense of justice inside the system, even if he didn’t always agree with the world government.
I think Garp believed he could still protect people and maintain peace by working within the marines. By staying, he could influence younger marines like Koby (I think he will become a very influential figure in the marines in the future (๑•ㅂ•)و✧) and pass down his version of true justice instead of abandoning the institution entirely.
Like instead of quitting and doing another thing, you change the system itself even though little by little. The marines wouldn't change unless it starts from the inside, not from outside forces.
His sense is justice seems to be broken, if it depends on defending slavers and murderers (￣∇￣") hopefully the Koby thing plays out and the marines wake up and turn against their masters
The more Garp appears in these flashbacks, the more I dislike him because why are you trying to win a morality game with pirates when youre a lapdog for war criminals （/TДT)/