'Thrill Kill' Soldiers: What Were They Thinking?

Five U.S. soldiers stand accused of using grenades and rifles to murder three unarmed Afghan civilians earlier this year, and investigators say several of the soldiers even collected the dead civilians' body parts.

In a videotape obtained by ABC News' Brian Ross Unit, one of the accused soldiers, Cpl. Jeremy Morlock, confessed to the murders. He said the officer in charge, Sgt. Calvin Gibbs, gave orders to carry out the killings and that Gibbs had no problem murdering innocent civilians.

Mental health experts overwhelmingly agreed the actions the soldiers have been accused of are inexcusable, and they said a number of complex psychological factors may play a role in why soldiers obey their commander's orders -- even when this means committing atrocities. The emotional toll of combat, people's tendency to do whatever they're told to do and the soldiers' fear of their sergeant, whom several of the them portrayed as a "thrill killer," could have contributed to their decision to kill unarmed civilians, they said.

VIDEO: Staff Sergeant allegedly went on a murderous rampage targeting innocent victims.Play
U.S. Sergeant Accused of 'Thrill Kill' in Afghanistan

"Sleep deprivation plays a role, there's some question of traumatic brain injury and some question about the use of prescription drugs," said Dr. Jon Shaw, a professor of psychiatry at the University of Miami School of Medicine who spent more than 20 years in the military. He has no involvement with the accused soldiers.

The attorney for one of the accused soldiers said his client was under the influence of prescription drugs during his videotaped confession. Another of the accused soldiers said drug use -- often hashish laced with opium -- was rampant at their base in Afghanistan.

VIDEO: Innocents Killed in AfghanistanPlay
'It Was to Cover Our Ass'

"There's a serious problem with substance abuse happening among our soldiers," said Dr. Jeffrey Victoroff, associate professor of clinical neurology and psychiatry at teh University of Southern California's Keck School of Medicine. He added, though, the he doesn't believe substance abuse alone led to murder.

Extreme stress, psychiatrists say, is perhaps one of the biggest factors that can affect soldiers' judgment.

"When you're exposed to that kind of stress, there's a readiness to be more passive and accept external authority, especially in a command structure," Shaw said.

'The Lowest Level of Morality'

VIDEO: Innocents Killed in AfghanistanPlay
'He Was Going to Kill His Mom'

"This is a very prolonged conflict and engagement, and there are multiple indications that these army units are worn out," said Dr. Paul Ragan, associate professor of psychiatry at Vanderbilt University Medical Center. Ragan, who served in Operation Desert Storm in the early 1990s, has no involvement with the soldiers.

Ragan said that in a war that's lasted nearly a decade, soldiers in combat could experience a wide range of emotions that influence their behavior.

"They may be suffering from mental fatigue, or may be feeling dispossessed and angry," Ragan said. "Are these men put in impossible situations [in which they] begin to dehumanize the other group and take their rage out on innocents?"

The emotional upheaval may also affect their moral judgment.

"In a group, there's regression to the lowest level of morality," Shaw said.

Role of Commanding Officer Difficult to Ascertain, Say Experts

The parents of another accused soldier, specialist Adam Winfield, said their son felt his life would be in danger if he reported Gibbs.

Mental health experts say that if the stories about Gibbs are true, the situation is very troubling and indicative of a military breakdown.

"There are a huge number of questions," Ragan said. "Where were the good order and discipline? Where's the supervision? It's possible that in a remote area with a fair amount of group-think and coercion, something like this can happen."

The famous experiment by the psychologist Stanley Milgram, in which subjects were ordered to administer a gradually increasing level of electric shock to other subjects who answered questions wrong. The receiving subject was actually an actor, and there was actually no shock administered at all. But the majority of the people in the study were willing to shock another person just because they were told to do so.

When the Blame Game Doesn't Apply

That coercion effect, combined with the multitude of psychological factors and a troubled commander may have led the soldiers to do the unthinkable.

"If you get one bad apple in an authority role, he could lead them to do almost anything," said Victoroff.

But Ragan said it may be difficult to sort out whether the soldiers truly felt as if they had no other choice or if they committed the murders and now blame someone else.

"The defense will have to demonstrate the extent of any kind of systematic intimidation, terrorizing and punishment," Ragan said. "What did the staff sergeant do when someone didn't go along with his orders?"

Shaw said that the soldiers should not blame the murders solely on their commanding officer.

"There are channels in the military where one can get help for this kind of commander who is problematic," he said.

Two of the soldiers said they were sure Gibbs would retalitate if they came forward with their concerns. In his confession, Morlock said he believed Gibbs would kill him, and Winfield's parents said their son also believed he would be killed if he came forward.

The Winfields told ABC News they tried to warn the Army and a U.S. lawmaker, but to no avail.

ABC News' Matthew Cole, Brian Ross and Angela M. Hill contributed to this report.