Slavery is bad, we all know that, even Masaru knows that, but remember that our MC is a NEET, and he even said a few chapters back that back in his world when he was a neet, he used to read web novels and stuff (probably isekai novels.) Although he knows slavery is bad, he doesn't think much of it since he's already seen it in novels/manga and is probably wondering how it'd be to have (a slave) one.
That's a very subjective point of view from some moral high-grounds.
In fact, slavery is not bad at all. Slavery only turns bad when the slave owner has too much money to buy and exchange slaves at an alarming rate.
But this interchangeability-problem also holds true for non-slave societies where you can just hire and fire without having to care about how the person you just fired is going to live from tomorrow on.
When I say that slavery is not bad at all, we have to look at how slaves are normally "made" from free people.
Usually, slaves are the spoils of conflicts/warfare. If we consider that the "winner takes it all" holding true, armies will always plunder/loot the regions they conquered, leaving the population there starving/without resources/tools to continue living.
However, imagine we're going to take slaves, then we take the population and the resources with us, resulting in them surviving (at least partially).
Slavery also reduces the amount of killing of foreign people, since they can be worth some money if they are enslaved instead.
And it reduces the amount of rape, as women who are raped are worth less or nothing or won't even survive the trip if they are "injured" in the process.
Slavery also makes it possible to sell your children instead of starving or killing them off, saving a lot of lives in the process.
And, while we're at it, you can also consider that, depending on the investment in the slaves, their owner will even pay for medicine/treatment to save his investment going down the drain with their possible death/disability.
Slaves are an investment, so mistreating them will result in a huge loss if they die or become disabled.
By the way, leaving aside the health issues, we also have a system that protects the slaves, as they are considered property.
As slavery can only exist in societies that have a certain degree of state regulatory system (law/proto-law) which defines the term of private property,
we have the security to assume that those people who are now considered property, are protected by the law of that state.
This is a huge improvement to their previous status, which was +- livestock or wild animals, as killing them in battle would've not being considered a crime for the state in conflict with them.
However, as they are now property to this state, hurting them will induce a fine to the assailant who dared to damage the property, meaning their social standard improved a lot [they went from "free game"-status to "touch me and be prepared to pay"-status].
Now, I'm not saying that from a modern standard slavery is something one should desire, but it bestows a certain "worth" to people who are living in a world without the concept of human rights.
Therefore it is quite a beneficial system for the people in a world without modern ethics.