Fort Riley ? An Army sergeant accused of a double murder took the witness stand Thursday to contradict a key prosecution witness’ version of events, breaking down as he described shooting a fellow soldier.

Sgt. Aaron Stanley, 23, of Bismarck, N.D., is charged with two counts of premeditated murder, stemming from the September killings of Staff Sgt. Matthew Werner, 30, of Oxnard, Calif., and Spc. Christopher D. Hymer, 23, of Nevada, Mo., at Stanley’s farmhouse in Clay Center, about 30 miles west of Fort Riley.

Prosecutors contend Stanley lay in wait for the victims, believing them to be informants for post police and wanting to protect an illegal drug trafficking operation. Stanley said he was trying to protect himself and a fourth soldier.

Werner had accused Stanley of having an affair with Werner’s wife and, Stanley said, had threatened to “cut a chunk out” of his face.

Werner and Hymer arrived at the farmhouse, Stanley testified, and he tried to leave. Hymer then shot at him.

Stanley, who served a tour in Iraq, said his instincts as a soldier took over.

“It’s all happening extremely fast,” he said. “It’s all split-second.”

Then, beginning to cry, he said: “Hymer tried to kill me, and I returned fire.”

Stanley said he went to check on the fourth soldier, Sgt. Eric Colvin, 23, of Papillion, Neb., and found Colvin wrestling on the kitchen floor with Werner, who was trying to stab Colvin with a large kitchen knife. He then shot Werner, Stanley said.

Stanley and Colvin have acknowledged that the home was the site of methamphetamine manufacturing and marijuana growing. Stanley said he was ashamed he had a large quantity of drugs around the home and attempted to get rid of them on the day of the shootings.

Colvin, testifying under a plea agreement with prosecutors, said earlier that he watched Stanley stand over the two victims and gun them down.

Instead of laying in wait for the victims, Stanley said, he hid in a closet. While he was armed, he hoped to avoid a confrontation, he said.

The defense rested its case Thursday evening, after calling 17 witnesses. Closing arguments were scheduled for Friday.

Thursday afternoon, the defense called Jim Pex, a forensic scientist from Oregon, whose testimony contradicted Colvin’s testimony about how the shootings took place and where the victims were when they were shot.

The defense also recalled a former soldier, Brian Robinson, who testified that he tried to calm Werner down before he and Heymer went out to the farm to confront Stanley and Colvin.

Defense attorneys suggested that Colvin has been inconsistent in statements made during various investigative interviews.

Colvin has acknowledged that he hoped to clear his name by taking the stand against Stanley. The government previously had portrayed Colvin as the mastermind of the events.

Post officials haven’t determined whether Colvin also will face a court martial, nor have they disclosed details of his plea agreement.

Stanley pleaded guilty Monday to drug use, drug possession, being absent without leave and adultery. His sentencing on those charges has not been set.

The AWOL charge stemmed from Stanley being unable to report to duty when he was in jail on drug charges. The adultery charge does not involve Werner’s wife.

All four soldiers were part of the 1st Battalion of the 41st Infantry Regiment, 3rd Brigade, 1st Armored Division based at Fort Riley. About 750 soldiers from the unit are concluding their second tour of duty in Iraq.

Both Stanley and Colvin were with Bravo Company and had served in Iraq when the unit was deployed for the early stages of the war.