When I said 'hero', I was just generally refer to any MC of a manga, whether they're truly heroic or not that's the usually the role they're written in. As stated, my confusion with the regular slavery thing (meaning, not specifically this manga) is how someone who's supposed to be from (near enough) our world with modern values is fine with it.
And I've no complaints about killing, we're all fine with killing if the situation calls for it. That's what armies are for in the real world, sometimes there's no way round it here or if you're suddenly up against the demon lord's army. If the MC was going around killing random people, yes, I'd view that as bad but I wouldn't be reading a manga with a MC like that.