A Marine looks at the blazing thatched houses of Duc An after a village raid on Aug. 6, 1965, during the Vietnam War.

In a riveting new documentary about the Vietnam War by Ken Burns and Lynn Novick, some U.S. veterans speak candidly about the human capacity for killing. They remind us that our reasons for killing don't have to be good. They may not even have to exist.

For instance, you can annihilate roughly 3 million Vietnamese during a 20-year war and neither you nor the rest of the country needs ever really know why.

Karl Marlantes, who served in the Marines, told the documentarians, "We're not the top species on the planet because we're nice. We're a very aggressive species. It is in us. People talk a lot about how well the military turns kids into killing machines. I always argue that it's just finishing school."

Throughout the 10-part series, we see massive antiwar protests as the immorality of the war pings the conscience of more and more Americans. Some of the largest demonstrations occur near the White House and at the Pentagon. In the end, we arrive at the Vietnam Veterans Memorial on the Mall, near the Lincoln Memorial.

More than 58,000 Americans lost their lives.

The documentary features the antiwar songs of the 1960s and '70s that accompanied many of the protests. But the real soundtrack is the rat-tat-tat-tat-tat gunfire of automatic rifles: the M-16 carried by U.S. troops and the AK-47 used by the North Vietnamese and Viet Cong forces.

Even if you haven't seen the documentary, you still know the sound. You heard similar rapid-fire shots coming from the windows of the Mandalay Bay Resort and Casino in Las Vegas on Sunday. Stephen Paddock, a retired accountant and high-stakes gambler from Mesquite, Nev., reportedly used a weapon similar to an AK-47 to kill at least 59 people and wound hundreds more.

It seems some of us are just as willing to turn the gun on fellow Americans as on our enemies in foreign lands.

Where did Paddock get a gun like that, you ask?

In the documentary, you'll see the United States sending massive amounts of military armament to South Vietnam. You might recall that the United States manufactures and exports more weapons than any other country.

We are the world's Guns R Us store. Surplus military weapons are now going to police departments — to be used for local law enforcement. Moreover, the United States has been at war or involved in some armed conflict with other countries for most of its existence — even when national security was not at stake.

Who can be surprised that a nation addicted to war would be awash in guns — and eager to use them?

In Vietnam, a village of innocent Vietnamese was massacred by U.S. troops. When asked by a TV reporter why he participated in the killing, one soldier just shrugged and said, "I don't know." Nobody was held accountable.

As for the tens of thousands of innocent people killed in the U.S. aerial bombing of Vietnam, it took years before America even took notice, let alone complained. In an interview with Washington Post writer Alyssa Rosenberg, Novick observed:

"Many, many people were killed by indiscriminate use of firepower, and that's bombing and artillery and naval battery and that kind of thing. And we don't consider those atrocities. That was just collateral damage. Unfortunate. Just what happens. And so there's something sort of inherent in how we waged the war that we have to think about as a society."

The documentary also showed that seemingly normal young adults could don Ohio National Guard uniforms and proceed to shoot and kill students at Kent State gathered for a nonviolent antiwar protest in 1970. In Jackson, Miss., we see bullet holes in a dormitory window at Jackson State where city police shot and killed two nonviolent student activists 10 days later.

Shockingly, a poll after the Kent State killings showed that more than 40 percent of U.S. residents thought the National Guard had acted appropriately.

John Musgrave, a Marine veteran, said that dehumanizing the Vietnamese made it easier for him to kill. Musgrave, who is white, called it "racism 101."

"This is war," former Marine Roger Harris said of the conflict. "Soldiers adapt. Killing, dying, after a while, it doesn't bother you. . . . I was made to realize that this is war and is what we do."

To read previous columns, go to washingtonpost.com/milloy.