The sight of human beings, most of them unarmed, being gunned down from above is jarring enough.

But for many people who watched the video of a 2007 assault by an Army Apache helicopter in Baghdad, released Monday by WikiLeaks.org, the most disturbing detail was the cockpit chatter. The soldiers joked, chuckled and jeered as they shot people in the street, including a Reuters photographer and a driver, believing them to be insurgents.

“Look at those dead bastards,” one said. “Nice,” another responded.

In recent days, many veterans have made the point that fighters cannot do their jobs without creating psychological distance from the enemy. One reason that the soldiers seemed as if they were playing a video game is that, in a morbid but necessary sense, they were.

“You don’t want combat soldiers to be foolish or to jump the gun, but their job is to destroy the enemy, and one way they’re able to do that is to see it as a game, so that the people don’t seem real,” said Bret A. Moore, a former Army psychologist and co-author of the forthcoming book “Wheels Down: Adjusting to Life After Deployment.”

Military training is fundamentally an exercise in overcoming a fear of killing another human, said Lt. Col. Dave Grossman, author of the book “On Killing: The Psychological Cost of Learning to Kill in War and Society,” who is a former Army Ranger.