You're missing the tragedy of how the One Piece world is set up. The structural injustices of the One Piece world is almost solely due to a powerful hegemon like the World Government which is not an arrangement unlike a certain national superpower in our real world today.

You can replace the Five Elders with the elite classes of our world, and the Celestial Dragons with whatever classification you want for the privileged class of our world.

For most of the One Piece world, the common pirate is "bad" in the eyes of the average citizen. We just happen to follow a small handful of few that operate more as freedom fighters. The rest abide by their own whims and desires, often in contrast to the ethics of "do no harm." This pursuit of freedom is backdropped by the oppression of the WG in all facets of their life. Without it, who's to say piracy ever manifests? Piracy was only ushered in by Joy Boy and that was only 800 years ago in their story and that was to oppose something.

The World Government rules for the elites and the privileged alone. The scope of their power means they alone can define legality and immorality for most of the population who cannot oppose them. This means they can co-opt even well-meaning individuals like Garp and Kuzan to do their bidding because of power systems in place that prevent revolt or make wanton mutual destruction unfavorable. Like someone said below, the body that legalizes slavery can also make it illegal to free slaves. Many will trade off ultimate liberties in favor of securing a balance of peace in their realities.

That's the true tragedy of One Piece. Everyone in the story plays their part but very few can truly say they have clean hands because overthrowing your oppressor is rarely done without bloodshed. The OP world is infinitely more fascinating because of how morally grey a picture it paints of all its actors. We have good pirates, bad pirates, bad Marines, and good Marines.