Certainly it will have been obvious to Netflix producers, I agree. I guess the point is that it may not have been burnt into the consciousness of the Korean writer in the way that is of us in the west. The Koreans have their own tragic history etched in their souls, as do the Cambodians, the Indonesians, the Rwandans and so on, as you suggest. Yes, it's a "special" kind of evil.
But you've made me reflect here, because probably for about 20 years I've been unable/unwilling to watch certain kinds of film, whereas prior to that I had no issue. I saw Schindler's List on release. I can't say I enjoyed it, but I found it moving, troubling of course. But I wouldn't watch it again. Nor any film of that sort that depicts things that actually happened. Even war films.
Fantasy violence? Sure, no problem, so that is probably why Squid Game is viewable for me. I didn't enjoy the killing and cremation, and I found the series profoundly disturbing, but in an a priori sense rather than empirically. That shit didn't happen. Somebody made it up, so I can cogitate on the metaphor without being troubled by realities.
I think the turning point for me probably was going to Cambodia in the 90s, and seeing the Killing Fields (actually, they were everywhere. There wasn't just one) and the S21 torture centre. Both were ordinary and banal, but horror lay not so far underneath. The S21 was a re-purposed school. Looked like any other city school. The Killing Fields looked like a park or a memorial garden, up until the point I tripped on something, looked down, saw that it was a bone poking out of the soil, at which point I noticed all the little fragments of fabric embedded in the soil, visible in between the blades of grass. I was standing right on top of mass murder.