You're probably aware of atrocities committed by American troops in Vietnam, such as the infamous My Lai massacre. But you're probably not aware of the atrocities committed by the North Vietnamese, for the simple reason that America has a free press which tries to expose such things, and Communist North Vietnam did not have a free press then, and doesn't now. That means documentary evidence, such as photos, of North Vietnamese atrocities is much less visible. This is also the case of the mass killings done by Mao and Stalin.
Why is there an endless stream of movies and books about the evils of Nazi Germany, but so few about the evils of Stalin's purges or Mao's Cultural Revolution, which were equally devastating? In large part because, unlike modern Germany, Russia and China have never become open societies, which means that their bloody secrets, like those of North Vietnam, have remained hidden.