I make the analogy with North Korea. North Korea is obviously a threat to South Korea. But if South Korea were to start bombing North Korea and kill 250 people a day the question would be how long it would take for people to ask "Why are you doing this?"
This is going to likely get me called a tankie, but the civilian massacres South Korea didn't cause a blink to the US. They killed whole communities that had a communist insurgency in them.
That was half the issue with the initial killings, as an aside. They were planned top down affairs. The dictator straight up gave quotas to local police saying "you need to find this many communists and register them for political reeducation" so the police just drummed up illiterate substinence farmers and had them sign they they were communists. Then they gathered them all up and shot them.
Because of the whole US involvement in the very creation of the south Korean state probably. Not that someone who has no idea about that is to blame for it since it's literally not taught in any way and even colloquially known as the forgotten war since less than a decade after it ended.