We don't live in a society that has casual slavery of the sort in Harem Hotel, so it's not really the same thing. Let's remember that one of the MC's other girlfriends has also been kidnapped, enslaved, put through hell and traumatized, and he's never seemed anywhere near as angry about it. To continue your example, if your girlfriend told you about the times in her past when she was enslaved and mistreated, would you be angry about it, or would you be as relatively indifferent as the MC?
I think his anger is perfectly understandable and reasonable, but his reactions were stupid, even if I understand them. I mean, I'd already guessed that Maria was half-elf, because of the scars on her ears, and I'm sure others had too; the only reason I'd not been 100% certain about is that as a half-dark elf, Maria should live to be about 250, and there was no indication from anyone that she'd aged more slowly.* So I thought letting her even go for the DNA test was a terrible idea; I also thought she knew/suspected she was half-elf, so her willingness to go seemed bizarre to me.
The MC's anger is because he has, on some level, been okay with elves being slaves; he's angry because he thought Maria was human, and seeing her treated the way elves are treated was the thing that made him snap, because he knows it's a terrible way to treat someone. And yet he was somewhat blasé about elves being treated that way.
*Common elves live fifteen times longer than humans, meaning a 1000 year old common elf is the equivalent to a 65 year old human, and they age at a similarly reduced rate (that's how Lin can be nearly 300 and look to be about 20). Dark elves life half as long as common elves, so they can live to be about 500, though probably less because they're not getting adequate healthcare, and age at about one-seventh the rate of a human. Half elves live half as long as their parent species, so half-dark elves should live to be about 250, and age at about one-third the rate of a human.
Therefore, Maria should either look a lot younger (like about 7 or so) or be a lot older (mid-fifties), and both would have been noticeable when she was growing up. Neither is the case. I think it was a math mistake; Runey says no.
It also proves the superiority of half-elves; elves live longer than humans but learn more slowly, while Maria is a very intelligent woman who was top of her classes, which proves she learns as quickly as a human, but will live substantially longer. Is this true for all half-elves?