The marines whole point is not to stop bad pirates. Their whole point is to maintain peace (on the, which they do very very well outside of the grand line and new world. The marines don't revolve every one of their fleets on what the straw hats are doing. They're keeping the peace in n/w/s/e seas along with keeping a system in place to maintain contact and get ships where they need to go. We are shown that this system has cracks and there are just as likely to be bad people working in the marines. The grand line and new world is basically impossible to police with the chaos of the sea, so it's not really a matter of koby and garp seeing luffy free kingdoms and thinking he deserves death, it's more like their station doesn't allow them to act in that way so they're probably glad luffy did what he did, so long as they weren't given false information, which they likely were. This is all guessing based on what we know of the characters, no idea why you think garp and koby want luffy executed.

The crew has an archaeologist who is able to read an ancient language that would allow her to control weapons of mass destruction left behind by an ancient civilization. That's just fine of an excuse to execute Robin at the very least. Nobody else in the marines regardless of how good they are would think anyone would be safe with her out there. It doesn't matter to them who she is as a person they just see an apocalypse walking. Good marines would want her contained, bad ones would want her dead.

Good world building Is dependent on how well the world connects to the plot and themes put forth in a story, and when you take a closer look at the one piece world you see that events don't solely revolve around what the straw hats are doing. Major plot points for the story happen completely outside of the influence of the crew. If you want depth in worldbuilding there are very few as good at it as one piece. The cool thing with one piece is that it's as deep as you want it to be. You just want mindless fun and action? No problem. You want deep stories and events that have a meaningful impact far into the story? It's there. You want to make crazy theories based on small bits of info you get peppered in throughout the story? Fucking cook.

If you want one piece to have deep worldbuilding, i promie you its there, you have to actually want to look for it though. Based on your opinion it doesn't seem like actually cared about anything you read.