The Khmer Rouge's ideology had almost nothing in common with the ideology of the Vietnamese Communists, or really any Marxist movement. It's difficult to see how an ideology that is intensely hostile towards all forms of industrial society and even towards the very existence of cities has anything to do with an ideology that has rapid industrial development as one of its central pillars. After the Vietnamese overthrew the Khmer Rouge, they put a very different type of government in place.

There were, of course, geopolitical calculations on all sides, which is why the United States and China supported the Khmer Rouge. I don't think moral revulsion alone often leads any government to declare war on another.