> something I don't really understand is that one nation killing another is more immoral than when a nation does this to their own domestic population.

I don’t know that anyone thinks a state’s violence against its citizens is less immoral. It’s more that countries are more hesitant to get militarily involved in the domestic affairs of another country because it would mean essentially declaring war against that state. But in a conflict between states, an outsider can more easily support one side militarily without declaring war against the other side.