In https://hpmor.com/chapter/42 , Harry mentally asserts that "Whoever had created house elves in the first place had been unspeakably evil, obviously," but his reasoning isn't given. I read Yudkowsky as assuming it's obvious to the reader, too. But it's not obvious to me! I'll pay you to present an argument that sways me from the position of "no, it's definitely fine, actually."
About the perspective you need to convince: I'm a roughly preference-utilitarian consequentialist, in a relatively unprincipled way that often looks a lot like deontology and takes the "just walk out" approach to the repugnant conclusion.
Most compelling argument so far: "It's probably not easier to create drudgery-loving elves than non-drudgery-loving ones; and it's bad to choose to bring into existence a worse version of a being than you could (as with drinking while pregnant, or choosing a trisomy during embryo selection)." Not satisfactory because:
It's not obvious that drudgery-loving elves are worse to bring into existence: arguably it would be worse to create elves that value the same things as other species, because they'd compete for the same inputs.
Having a policy of "X (though it's not actively bad and has clear benefits for you) is forbidden because Y (which doesn't benefit you) is morally superior" will lead to many people simply not doing X, reducing welfare. (Here, X is "create drudgery-loving elves" and Y is "create non-drudgery-loving elves.")
If I remember right Eliezer himself commented on this issue at the time, and concluded that something like that could be done right (that is: it would be ok to create a person who sincerely likes boring but necessary work) but the way it is specifically done in the HPverse was messed up: elves are by design enslaved and sort of powerless and abused all the time. Elves more or less aren't allowed to leave cruel masters who mistreat them, and whoever made them *that way* did something wrong.