Page 18 Why would he say that when he was so reluctant to join another party at first? He changed his mind when he found out he would own a female elf slave?
There's some creepy subtext in these betrayed by party plots. The MC keeps talking about obedience, and they usually go down the route of slavery or taming monsters and of course it's always women. Heaven and Earth move to make them look more righteous, but the MC always needs to have complete domination over romantic interests or their from a race that's treated as subhuman by everyone else. It's like a crippling victim/inferiority complex and you can't even conceive of interacting with a woman without having some power over them. I could only think of a few of these betrayed/revenge plotlines where the party members aren't subordinate or at least don't worship the ground the MC walks on. Even then the MC still has to be flawless and overpowered.
https://mangadex.org/title/87a756b4-e4a9-4d38-81a1-d14c8fc90baf/
https://mangadex.org/title/30eb9ee7-4cfe-4036-95f9-4aac5e05b505/
https://mangadex.org/title/0b171f64-89a5-4c37-b5f9-75cca57e8787/
I get it's suppose to be a power fantasy, but I genuinely don't get the appeal of subjugating people much less romantic interests. Wouldn't there always be a lingering thought that you're just taking advantage of them? Kind of a downer.
Eh, it's not inherently a problem at all.
The problem is more like an author's own psychological issues about wanting to feel like "an hero" by being "different from the rest". Doesn't speak well of them.
The only stories that I can think of off the top of my head that it makes natural sense in is stories in which the MC's betrayal very clearly scarred him and resulted in him becoming severely paranoid about being betrayed.
I think you're misunderstanding something, besides this stuff. In real life, slaves tended to actually be surprisingly loyal to their owners. Manga and Hollywood
heavily exaggerate how they were treated. You underestimate how strong the human instinct is to follow a leader and feel like part of a group. Just look at how easily people end up in something as common and simple as an echo chamber online as an example and that isn't even in person. And when in a group, especially a small one, even as a slave you subconsciously will develop an "us vs them" view of those outside of the group. Sure, there's places in real life in which slaves are treated terribly, but manga takes clearly from western slavery and in the west the mistreatment of slaves was rare both because of their expense and because westerners viewed them more like unpaid servants whereas other places viewed them as basically non-human objects to vent their sadism on.
Also, you seem to have a weird moralization of power and dominance and subordination. There's nothing immoral or moral about such things. They're completely unrelated to morality. I have no doubt that you would want to be the one with the power, but that you'd say it's okay when you do it.
Oh, and the elf slave thing is a blatant lie. He was asked what kind of people he would form a party with and he told the guild girl, who introduced him to the elf when he accepted her suggestion. He didn't know she was an elf or a slave or a female when he accepted the suggestion. Not that it even matters. I'm sure your "morals" would be soothed if he learned about her and decided to just leave her to anyone else who might end up owning her even though he knows that he could have helped her. Because you're
such a good person.