As a show watcher, the show never really gives me a reason to hate Marco outside of hurling rocks at Earth, a place we rarely ever see. Like, everything he's said about the treatment of Belters by the "inners" is correct. And, once the Ring Gates were opened we saw once again Earth and Mars try and screw Belters out of a fresh start; ensuring the continuation of those on top being on top and those on bottom staying on bottom. He's able to see where this is all heading and gets out ahead, along with the Mars faction and establish themselves before Earth realizes that the entire paradigm has shifted.
Honestly, it makes Earth looks really dumb. But, I can excuse that because those on top often don't realize that they can be easily replaced. Once the Ring Gates were opened and it was discovered that thousands of habitable worlds exist, Earth's importance and power became greatly diminished. Yet, Earth never seems to even discuss this. The reason the Belt had to take shit from Earth is because all primary resources (water and food) came from Earth. They had the monopoly on those resources as being the only habitable planet, so no one could really fuck with Earth. But once the Rings opened up access to these resources, Earth's days of importance were numbered.
As a Belter, why would you really be opposed to Marco outside of his terrorist attack? Earth isn't going to give you a fair share of resources or access to habitable ring planets. Rather, they will ensure the status quo remains with you on the bottom. I'd fucking throw in with Marco too.
That's well and good, but Fred was still an important character. Yet, we see no shift in the political situation caused by his death. Sure, Belters can think that about him, but you should show that. And regardless, he was a major leader in the Belt. His assassination should create a large shockwave in regards to the balance of power within the OPA. But, we don't see that and Marco just gets to declare victory without doing any of the narrative work.