Communism

The US backed him and portrayed Vietnam as imperialists for fighting against him, so that should really tell you most (though not all) about what you need to know.

I don't understand how someone can be a communist and an ethno-nationalist simultaneously.

workers of the world unite... separately.

Pol Pot managed to killed maybe a maximum of three million people. This is nowhere near as prolific as the death toll that Joseph Stalin had.