Ever since S.L.A. Marshall did a study during WWII where he discovered that the majority of infantrymen were really sort of extras, and most of the direct man-to-man fighting was done by a small number of them, armies have spent a lot of time just training their troops to shoot at people. Humans are not by nature killers of fellow humans.
On the other hand, I'm fairly sure I know what the primary motivation behind this is. It's actually stupidly simple when you think about it. Nietzche was right. Fight monsters and you want to kill them. And from where they're sitting, they're fighting monsters, people who kill indiscriminately, who obey none of the rules of civilized warfare, who neither ask nor offer quarter. This is not a unique occurance. It happened with Allied forces fighting the Japanese in WWII, it happened in Vietnam, it's probably happened in every large-scale guerilla conflict since the dawn of the 20th century.