Zembla has a post excerpting from a former Army ranger and West Point professor that goes into how the Pentagon brainwashes soldiers into feeling ok about killing.

The reality is that the brains of human beings -- unless they fall within the demographic sliver we call psychopaths -- are hardwired not to kill other humans. ... The only thing that has any hope of silencing the midbrain, he argues, is what influenced Pavlov's dogs: conditioning.

The need for new drills became apparent once researchers noted that a majority who had been trained in other ways to kill, surreptitiously refused to do it.

...
In World War II, when U.S. soldiers got a clear shot at the enemy, only about 1 in 5 actually fired, according to sensational and controversial research by Army historian Brig. Gen. S.L.A. Marshall. It wasn't that they were cowards: On the contrary, they performed other perilous feats, including running onto the battlefield to rescue fellow soldiers, and sometimes they even placed themselves in greater personal danger by refusing to fire. And yet at the moment of truth, they just couldn't kill . . . .

The Pentagon improved firing rates. Research suggests that 55 percent of U.S. soldiers fired on the enemy in the Korean War. By Vietnam that rate had climbed to more than 90 percent. Police studies document similar changes in recent decades . . . .

Er...go Pentagon? Thanks for turning people into crazy killing machines so that you could remain relevant and keep your stinking jobs.