In WWII, the Japanese and Germans were incredibly effective killers because they were taught that Americans were sub-human, not real people with wives and families.
The Allies did this too, some militay experts say that it's absolutely necessary to dehumanize the enemy. . .
It was important to de-humanize the Italians, Germans and Japanese so that American soldiers would feel good about killing them.
To Wit, view this Walt Disney WWII cartoon, a timeless classic showing how the media can dehumanize the enemy using propoganda: