His fiancée says Army Staff Sgt. Rashad Valmont quit eating and put in extra hours at the gym and sauna after being ordered to shed 3 percent of his body fat in a hurry. Two days later he walked into a superior's office holding a Glock handgun, took aim and fired six shots, killing the man.

More than a year later, a military jury hearing Valmont's court-martial in southeast Georgia must decide: Did the accused soldier kill his master sergeant to exact revenge for poor performance reviews? Or was Valmont, as his defense attorneys insist, dehydrated and delirious from a crash diet forced on him by a different supervisor?

"He just was down and he felt like his superior officers wanted him to fail," Marina Nichols, Valmont's fiancée, testified at his trial Wednesday. "He was exhausted and he looked really drained. He didn't eat anything. He actually threw up."

Valmont, 30, of Jonesboro faces a mandatory sentence of life in prison if convicted of premeditated murder in the June 2010 slaying of Master Sgt. Pedro Mercado. Both men belonged to an Army Reserve unit based at Fort Gillem outside Atlanta. The Army Medical Department Professional Management Command helps with the recruitment, retention and readiness of medical units and personnel.

Valmont's court martial began Monday.

Co-workers testified that Valmont, on top of a yearlong struggle to lose weight, was already stressed after getting a poor performance evaluation and receiving several corrective counseling sessions from his superiors. They say he spent much of his time at work in the days before the shooting working on a rebuttal to his evaluation.

On the afternoon of June 17, 2010, Valmont went to his car at Fort Gillem and retrieved the Glock pistol he kept in the driver's side console. He walked in on Mercado while he was talking with a civilian co-worker.

"Staff Sgt. Valmont entered his office and opened fire," Anthony Williams, who worked with both men for about six months, testified. "I've never seen Staff Sgt. Valmont look like that. His eyes were blood red."

He said he asked Valmont why he'd shot Mercado and was told: "I'm tired of him. (Expletive) him."

Then Valmont walked out, left the building and drove to a nearby police station. Prosecutors say he turned himself in and told officers: "I just shot someone at Fort Gillem."

Prosecutors say the killing was Valmont's cold, calculated retaliation against higher-ranking supervisors who were trying to help him perform his job better and were willing to discipline him if necessary.

Mercado had also recently turned down Valmont's requests for leave and vacation time.

"Master Sgt. Mercado initiated a number of personnel actions toward Staff Sgt. Valmont," said Capt. John Riesenberg, one of the Army prosecutors. "So he had a rational motive to be violent."

But Valmont's defense team insists he grabbed his gun only after being driven to exhausted delirium by fasting. Days before the shooting, he was trying to get a supervisor's approval to attend an advancement course the following week.

He met the minimum weight requirement, barely. His direct supervisor, Sgt. 1st Class Tracy Mosley, wrote that she worried about Valmont gaining weight before the training course began, so she ordered him to lose an extra 3 percent of his body fat before he could go.

Valmont's fiancée testified that he came home that night weary from a body wrap at a sauna. He skipped dinner and went to the gym for several hours instead. She heard him vomiting after he returned home. The next morning, the day before the shooting, he went straight back to the sauna before work.

Psychiatrists at Walter Reed Army Medical Center reviewed Valmont's case file and interviewed him before concluding the crash dieting left him delirious.

"There's some risk of violence with delirium," said Navy Cmdr. Janis Carlton, one of the psychiatrists who diagnosed Valmont. "All the indicators were that he was depriving himself of food and water and had been sweating and exercising."

The court-martial was moved from Fort Gillem to Fort Stewart about 40 miles southwest of Savannah. The trial was expected to continue Thursday.

Mercado's family members say they want to see justice done a year after his slaying. After 30 years in the Army, including deployments to Iraq and Afghanistan, the 47-year-old master sergeant was finally preparing to retire. His two older brothers, each wearing one of Mercado's gold rank insignia pins, say he had a high-paying job as a consultant all lined up.

"My brother was a good man. He was trying to help this guy when it happened," John Mercado, one of the brothers, said outside court. "It's very hard. It's been a long year."