I don't think that's the message the author was trying to convey. I think the message was "forcing radical viewpoints on others without looking at the bigger picture is stupid." (You give the word "radical" a negative connotation, but that's wrong. Radical just means heavily departing from what is considered the norm.)
Because that's exactly what that "hero" was. Heavily against what was considered the norm. And trying to force his viewpoint on everyone else by going "you follow my way or I will use violence" without looking at the long-term impact his actions would have.
A lot of those slaves weren't merely indebted servants. They were often criminals serving a sentence. Which we even still do today as penal labor. Though conditions for that are far less harsh in our world of course. So what would happen if those criminals suddenly got freed? Most likely wouldn't suddenly become upstanding citizens. So what that hero was doing would likely cause more damage than do good in the long term.
Lets also not forget that Minato said he's also against slavery. But that the way the hero went about it to "put an end to it" was very naive, short-sighted and selfish. I feel like the idea the author was trying to convey was that if you think the system is wrong you shouldn't cause a big commotion in the now without looking at the bigger picture. That if you actually want to make a difference you have to change the system from within, which takes effort and time.
Sigh. Okay. I'm going to get up on a soapbox here because I'm frustrated. I really hate the way morals are presented in this story. This story takes place in a fantasy world where slavery is legal. Specifically slavery of criminals and the children of those who cannot pay their debts. Enter this party called Blue Justice. Here's my problem with Blue Justice: While I do believe they think they are doing the right and moral thing, it seems to be in a very small, but performative way. My personal opinion is that slavery is wrong no matter what the circumstances are because, in short, it takes away the bodily autonomy from an individual. So while I agree with the Blue Justice party that slavery shouldn't be a thing, THEY ARE GOING ABOUT IT IN AN INCORRECT WAY. Trying to free an individual or a group of slaves you come across is all well and good, but what happens after you do? Where will they go? Will they be able to function in society if they were 'freed' from slavery unlawfully (such as buying a house or land, getting a job, getting a marriage license, etc.)? It all seems extremely short sighted and inefficient. The problem isn't with the individual slave sellers, but rather with the system itself. But they aren't trying to change the SYSTEM, they're just trying to enforce their morals with no laws or lawmakers backing them which is why they have such a horrible reputation.
I feel like this manga is trying to say 'slavery is okay; it's not that bad' by showing slaves that are okay with not being freed even though we know that if they don't give the masters absolute obedience, they can be punished with whips and lashes, or worse, rape. It's so very obvious that the system in this world is broken, but what the Blue Justice party is doing, while well-meaning, is useless beyond the few people they're able to impact during the short amount of time they're at any given location.
So in conclusion, they are short sighted, inefficient, ignorant, and arrogant. Which is a hell of a way to portray the only characters in the story with a strong sense of morals. (-_-) I can't say I'm impressed with that choice as a writer. I sincerely doubt this story will have any sort of arc detailing the dismantling of the slave trading system, so it seems these characters will be used as an example os stupidity. I'm not sure what the message was the author was trying to get across, but I sure hope it isn't 'people with strong moral compasses are stupid and annoying' because that's what's coming across so far. The fact that they are portrayed as characters that don't listen to other's words (like the slave when she said she didn't wish to be freed) and seem to be relatively inarticulate when it comes to actually having a constructive conversation about it with someone is discouraging. When they were first introduced, they were described as, and I quote, "radical". I don't think it's "radical" to believe that everyone should have freewill and bodily autonomy. I believe it's a basic human right. The fact that this story is using the symbolism of strong morals=stupidity is not amusing to me.