This proves exactly what I mean about how the writing for the Marlyens and the world generally could have been better.
"But realistically, none of Marley's supporters are understandable. It's pretty much just mindless racism, blaming people for the sins of their ancestors."
I don't mean to bully in any way, but that's exactly one of my main points. The fact that people treat this so casually or like it's not a big deal is a giant part of the show's fault. This isn't our modern equivalent to the Roman Empire, to Vikings, to Ghengis Khan campaigns that we reminisce like old stories. This is 2000 years of constant and relentless oppression by a "singular group of people." That is not even close to mindless racism. If anything, I feel like it's expected for the world to hate them. Additionally, this isn't "ancestors" like 200 or 300 years removed. The Great Titan War ended 80 years ago. So people from that era are still alive. It's basically like how we treat the holocaust.
Lastly, I'm not saying Eren and them did have that many options, but they were actively putting themselves in a horrible position. First, everyone in Paradis is way too ignorant; these people had their memories stolen, so I get it, but their citizens actively hate the world because they hate them for completely justified reasons. They don't really know the history that the people of the world had gone through. Grisha is their only source through his journals, and he is the definition of a biased narrator; he hated the world, so his writing would very much reflect that. And he preached a nationalistic uprising by Eldians, which they have no right to demand of anyone on the outside, considering their history.
For example, some Nazis survived WW2 and went off to an island. They have kids and spread their beliefs, but leave out all the horrible atrocities they committed. 80 years pass, then people from the outside world find them. These people start spouting all the hateful things that were taught to them, not knowing of the context that they come from. The rest of the world is rightfully horrified. These people know nothing else, so to them the world hates them for their "just" beliefs. So naturally they become hostile. As the rest of the world, you have an option: let these people continue to exist and have their disgusting beliefs, or do something to stop them.
It's not a perfect analogy, but you get the idea.