“This particular night, it was December 2003, there was, I’d say, more than 100 targets,” Mr. Sepi said. “Each little team had a list. And at this one house, we blow the gate and find out that there’s this guy sitting in his car just inside that gate. We move in, and he, like, stumbles out of his car, and he’s on fire, and he’s, like, stumbling around in circles in his front yard. So we all kind of don’t know what to do, and he collapses, and we go inside the house and search it and find out it’s the wrong house.”

Although Mr. Sepi said that he felt bad at the time, he also knew that he had done nothing but follow orders and that the Army had paid the man’s family a settlement. He did not imagine that the image of the flaming, stumbling Iraqi civilian would linger like a specter in his psyche.

Listening to Mr. Sepi recount the story of a death that he regretted in Iraq while grappling with a death that he regretted in Las Vegas, his lawyer grew determined to get him help. “It was just so shocking, and his emotions were so raw, and he was so messed up,” Ms. Lemcke said.

An Unusual Legal Deal

She found compassion for him among the law enforcement officials handling the case. The investigation backed up Mr. Sepi’s story of self-defense, although it was never determined who fired first. It made an impression on the police that he was considerably outweighed  his 130 pounds against a 210-pound man and a 197-pound woman. And it helped Mr. Sepi that his victims were drifters, with no family members pressing for justice.

The police said that Kevin Ratcliff, 36, who was shot and wounded by Mr. Sepi, belonged to the Crips and was a convicted felon; Sharon Jackson, 47, who was killed, belonged to NC, the Naked City gang, and an autopsy found alcohol, cocaine and methamphetamines in her blood.

Buoyed by an outpouring of support from Mr. Sepi’s fellow soldiers and veterans’ advocates, Ms. Lemcke pressed the Department of Veterans Affairs to find treatment programs for Mr. Sepi. This allowed an unusual deal with the local district attorney’s office: in exchange for the successful completion of treatment for substance abuse and PTSD, the charges against Mr. Sepi would be dropped.

After about three months in jail, Mr. Sepi spent three months at a substance abuse program in Prescott, Ariz., in late 2005, where the graying veterans presented an object lesson: “I don’t want to be like that when I’m older,” he said to himself. In early 2006, he transferred to a PTSD treatment center run by the V.A. in Topeka, Kan., where he learned how to deal with anger, sadness and guilt, to manage the symptoms of his anxiety disorder and, it seems, to vanquish his nightmares.