Robert Bales did unspeakable things. As a staff sergeant in the U.S. Army serving his fourth combat tour, Bales suddenly went berserk on March 11, 2012. He crept out of his base in the Kandahar province of Afghanistan, stole through two villages at night and systematically slaughtered 16 civilians, including women and children.

Then he turned himself in, but not before setting his victims’ bodies ablaze.

Bales’s murderous meltdown remains arguably the worst atrocity of the Afghanistan war. In 2013, he admitted to the killings and was sentenced to life in prison.

But it also remains one of the most mysterious moments in a conflict approaching its 14th anniversary. When asked by a military judge in 2012 why he killed children the same age as his own kids back in the United States, Bales replied: “I’ve asked that question a million times, and there is not a good reason in the world for the horrible things I did.”

In the absence of an explanation, various theories emerged: Bales suffered from post-traumatic stress disorder; he was pumping steroids; he was strained from excessive deployments; he was under extreme financial stress with two homes underwater back in the United States; he had snapped after witnessing another soldier lose his leg the day before.

According to one article, the problem wasn’t Bales but the anti-malarial pills he was popping. “Mefloquine can mess with the mind in more serious ways, causing confusion, hallucinations, and paranoia,” Wired reported. “I like to say this drug is like a horror show in a pill,” Remington Nevin, a doctor, told the magazine.

But in a letter written by Bales disclosed recently, he suggests that the source of his horrors was simpler still: war itself.

“My mind was consumed by war,” Bales wrote to a senior Army officer late last year as he made a plea for clemency. “I planted war and hate for the better part of 10 years and harvested violence.”

Thousands of other Americans have served in combat without snapping and slaughtering innocent civilians, of course. But in his letters, Bales portrays himself as a regular guy who lost his way in the fog of war.

“After being in prison two years, I understand that what I thought was normal was the farthest thing from being normal,” he wrote according to the Tacoma News Tribune, which posted Bales’s entire eight-page plea for mercy online.

A former stockbroker from Ohio, Bales signed up for the Army shortly after the Sept. 11, 2001, attacks on the United States. He had already served three tours in Iraq by the time he was sent to Afghanistan in late 2011. By then he had also begun to show signs of the mental and physical toll of the war: a drunken-driving arrest in 2005, a hit-and-run accident in 2008, a head injury suffered when his Humvee overturned in Iraq.

Although his superior officers considered Bales to be a reliable noncommissioned officer, a soldier under his own command considered him “paranoid” and even “crazy,” according to the News Tribune.

In his letters, Bales explains how his mind unraveled in the theater of war.

“I still don’t believe the PTSD, [traumatic brain injury], alcohol, steroids, and sleeping pills were the only factors in my actions on 11 March 2012,” he wrote to Lt. Gen. Stephen Lanza. “I believe there is much more. I worked hard wanting to be the best combat Soldier in the Army. I was obsessed with studying manuals, watching war documentaries and reading about insurgencies. Later in my Army career, I began to change. I didn’t want to make a bad decision on the ground and [lose] one of my guys. We trained hard and most of the guys understood why we trained hard. … I had been through fire fights, IEDs … mass casualty situations, and many other combat situations. So, going into my fourth deployment, I was experienced.

“Normally that would be a good thing, but now I know it made me paranoid and ineffective,” Bales wrote.

“You relive every engagement and try to learn from it, but you’re stuck,” he wrote. “If I only noticed the road being free of people, I might have noticed the IED. If only I had taken the shot, [Name Redacted] might have his leg. The list of if I only had done this it might have turned out differently is endless.

“It is not if you and your friends are going to die, but when,” he wrote. “I became callous to them even being human, they were all enemy. Guilt and fear are with you day and night. Over time your experiences solidify your prejudice. The IED blast in the populated neighborhood proves that they are all bad. Someone had to see something. A huge hole was dug and a 150 pound drum of explosives was placed into the ground. The rockets, mortars, IEDs, RPGs, snipers and pop shot guys you can never seem to catch frustrates you. It is like the school yard bully who punches you every day and gets away with no recourse. The hate grows not only for insurgents, but towards everyone who isn’t American. My mind was consumed by war. I planted war and hate for the better part of ten years and harvested violence. After being in prison for two years, I understand that what I thought was normal was the farthest thing from normal.

“War is evil,” he wrote. “I see that now. I, being part of war, acted in evil.”

Bales also revealed that he has been baptized and become devoutly Christian since his incarceration. He prays for the souls of those he killed. He is attending barber school. He is in anger management. He takes medications. He voluntarily cleans the latrines.

His letter includes banal descriptions of his longing for his old life: taking his kids for a hike with their dog, playing soccer with his son, fishing. At times, it seems as if Bales truly believes he could one day be forgiven for killing 16 civilians.

“You have the ability to set me free to be a father, a husband, a neighbor, a friend,” he wrote.

But the end of his letter is as brutal as the reality of his sentence. (Lanza rejected Bales’s clemency bid in March, long before the letter was made public. The Army Court of Criminal Appeal may take up his case, but it is unlikely.)

“Over my past two years of incarceration, I have come to understand there isn’t a why,” Bales concluded his letter. “There is only pain.”