After World War 2, a military study concluded that only a quarter of American soldiers were willing to line up their sights on a human target. To combat this problem, the army found ways to dampen our natural aversion to killing one another. They replaced bullseye targets with human silhouettes, made training conditions more similar to actual combat, and trained recruits to avoid taking personal responsibility for killing.

After enough training, you can eventually learn to stop seeing people at all, and just see silhouette targets everywhere... even in the mirror.