CAMP LIBERTY, IRAQ, Sept. 27  An Army sniper is taught to kill people “calmly and deliberately,” even when they pose no immediate danger to him. “A sniper,” Army Field Manual 23-10 goes on to state, “must not be susceptible to emotions such as anxiety or remorse.”

But in a crowded military courtroom seemingly stunned into silence on Thursday, Sgt. Evan Vela all but broke down as he described firing two bullets into an unarmed Iraqi man his unit arrested last May.

In anguished, eloquent sentences, Sergeant Vela, a member of an elite sniper scout platoon with the First Battalion, 501st Infantry Regiment, quietly described how his squad leader, Staff Sgt. Michael A. Hensley, cut off the man’s handcuffs, wrestled him to his feet and ordered Sergeant Vela, standing a few feet away, to fire the 9-millimeter service pistol into the detainee’s head.

“I heard the word ‘Shoot,’” Sergeant Vela recalled. “I don’t remember pulling the trigger,” he said. “I just came through and the guy was dead, and it just took me a second to realize the shot had come from the pistol.”