It's stupid because it's more Japanese manga isekai slavery nonsense, but the plan here, as dumb as it is, makes a twisted sort of sense
The racist asshole guy pushes the slave liberation movement to create a publicly acceptable way to further crush the slavery business, forcing slavers to essentially "liquidate" their inventory (kill the slaves) and thereby get rid of the demi-humans that he hates so much.
Based on the conversation here, the minimal rights of slaves don't protect them from being killed for balance sheet purposes, but do stop them from being sold into experimentation or whatever. It's a bit counter-intuitive but I suppose it's not that much different than, say, an animal shelter that is prohibited from selling its animals to laboratories for experimentation but is allowed to euthanize animals that haven't been adopted after a certain period on the basis of freeing up space/resources for incoming animals that do stand a chance of making it out.
So Mr. Racist Twatwaffle hits on an idea: essentially commit fantasy insurance fraud in theme if not in actual execution. Round up the demi-human slaves from slavers looking to cut their losses on the understanding that they'll be killed and everything is legit, but in actuality they take those slaves and sell them to other shady businesses that would not be able to legally acquire them. Douchebag monster guard captain noble guy or whatever he is then gets what he wants with a bonus: the demi-humans are no longer present in the country so that he can have his stupid racial purity nonsense, and he gets rich(er) off of it.
In a real-world sense it's like a thief stealing the inventory (let's say... couches) from a warehouse to sell on the black market, then setting fire to the building to cover his tracks and make the owner think the inventory was lost in the fire instead of stolen. Except in this metaphor our thief is also racist against couches for some reason.
It's still quite dumb, needlessly complicated, and wholly unnecessary, but I think it makes more sense than it's being portrayed as.
The only part that's hard to reconcile is what the kingdom's plan is if they deal with these shady, racist nonces. Because presumably crushing the people profiteering illegally off of slave disposal would still mean that the slaves are destined to be killed to take the financial write-off by the slavers anyways.
They could've framed it as the racist pricks say they're taking the slaves from the slavers to set them free elsewhere but that's a lie and they kill them/sell them. It accompishes the same goal without the over-engineered bastardry, but nobody said slop manga writers were particularly smart/clever.
You need to understand something, japan had little to no examples of slavery in it's history before the sengoku era and was generally treated by most people with contempt at the time, japan did have something similar to indentured servitude but that's not slavery but debt repayment, my point is your expecting a culture that does not have the same concept of slavery that the west has (though to be fair, the west's take is also a bit artificial thanks to hollywood), so slavery doesn't have anywhere near the same impact to the japanese that it does to a westerner.
I've made this same argument in other manga to explain the reason Japan treats slavery as just a weird isekai social quirk that the MC is willing to accept or as something the slaves can romanticize about and not want to be freed because it shows their devotion and love for their master. But it's also worth noting that while they didn't have outright chattel slavery for a long, long time, they have done a bunch of things that are pretty indistinguishable in function but are only protected by less severe euphimisms or language made to create deniability. Indentured servants could easily have the system gamed against them so that they had no chance of repaying their debt and only had the absolute minimal rights necessary. There was also comfort women, working POWs to death, and human experimentation on non-Japanese during WWII.
It's not so much that they didn't have chattel slavery and therefore have no context for seeing the horror of treating people as sub-human or property, it's that they found a way to dress it up, change the terminology, or plausibly deny its existence in order to create the illusion of things being better (as opposed to the west just deciding to be OK with it because the racist attitudes were publicly acceptable up to a point.) and Japan's natural cultural tendency towards non-confrontation and problem avoidance means that nobody wants to pull at those strings because it would look bad and cause issues that would need to be dealt with.
the lesson, of course, being that everybody sucks, just in different ways.
also in hindsight this dumbass manga was in no way worth all the effort I went to to type everything above. But it's too late now.