So, the original post he was quoting was going "I don't get how they can torture people like this and not get that they're the bad guys" to which he said "it's their culture". This isn't about him saying it's okay/good, it's about these people being brought up in a society where it's normal, so they're likely to not ever think otherwise. If you're raised in a society where torturing another race is commonplace+normal, it wouldn't be surprising to see you end up also torturing that other race when you grow up, regardless of if an outside observer would think it's cruel.
People like to think that their moral compass is entirely their own and that even if they were raised elsewhere they would be just as good a person, but it's just not true. Upbringing is a huge part of what forms your moral compass, it's why nobility(and the modern mega-rich) have less empathy for the lower class, it's why slavery was so accepted 300 years ago. These people are not fundamentally, biologically, predisposed to being worse people. They're just raised in environments that turns them into them.
Back to the manga: Obviously, we get that these dark elves are doing something evil, that's the point. But a dark elf raised to think "the forest elves are lesser, and they're the bad guys, so it's our right to do with them as we please" isn't likely to think twice about doing these things + think of themselves as the "evil" ones.