Answer:
During wartime, countries will aim to politically and socially dehumanize the enemy. It's to illicit an emotional response of hatred, disgust or fear out of the general public over another population. When militaries dehumanize the opposing side, the soldier's are more likely to pull the trigger. Similar events happened in Vietnam, the Vietcong were presented as barbaric animals which lead to a plethora of atrocities committed against North Vietnamese civilians by US troops.
Side note:
I don't know if you any of you are fans of the British show Black Mirror but, this issue is presented perfectly. In the future soldiers and lenses over their eyes that turn people into looking like zombies. The soldiers kill the zombies because their is no problem in killing a creature. When they take the lenses off, they realize their massacring innocent human beings and have been tricked. Episode is "Man on Fire."
