Slave waifus means trashy manga because author/mangala too lazy to develop relationship. Slave = instant waifu
At the very least, he actually had morals.
Even if using the (abhorrent as-implemented in stereotypical fiction) practice of slavery as a plot device -- a majority of these JP WN/Manga writers have absolutely no qualms about MCs that leave them slaves, out of their own insecurity. As both MC character and JP Author.
That's what I hate the most: characters "from modern Japan" that behave like total sociopaths the moment they are placed in "another world". (You or I would NEVER behave like that. We'd try to find them an out ASAP, as long as it wouldn't get us killed. To varying degrees depending on person.) Author wants to say it's a parallel set of morals necessary in another world? Fine. At least illustrate the process of MC adapting to it, instead of assume it as a given. Or worse, with idiotic reason lilke "they're happier that way (as slaves)".... as the person is literally suffering and begging for help in front of them.
This MC apparently has proper modern ethics.
Doesn't want to get involved in sticking his neck out and risk dying himself. Author immediately backsplains why MC didn't release her immediately. Gives some context of how he's adapted to the morals of this isekai world (his master speech).
Only thing that could make this better is a side note that the Blue Goddess takes retribution regularly for her "favored creatures" being made slaves and abused by humans. Rounding out the religious/mythical plot.