Pittman called 911 to report that her ex-boyfriend was pounding on the front door, said Pennsylvania State Police Trooper Anthony Manetta, who investigated the shooting.
At the same time, Atkinson armed himself with Pittman's gun, which was next to the bed, and waited in the hallway to confront the intruder, Nelson said.
Barbour kicked in the front door, which had been secured with a dead bolt lock, and rushed up the steps toward the bedroom, Nelson said.
As Barbour advanced down the hallway, Atkinson fired seven shots at him, he said.
Despite his wounds, including one at the neck, one near the left arm pit, one in the center of the chest, and two in the back of his left arm, Barbour continued advancing and ultimately entered the bedroom. As he approached Pittman, Barbour collapsed, Nelson said.
He was pronounced dead at the scene.
"I think most reasonable people faced with that situation would fear for their lives," said Nelson.
Barbour was not armed, police said.
According to Franklin County Coroner Kenneth L. Peiffer Jr., Barbour's blood-alcohol content at the time of death tested at .26.
The legal intoxication level is .10.
Nelson explained that the Pennsylvania Crimes Code states that if a reasonable person is in fear of his life or in fear of personal injury, deadly force can be used for protection.
Nelson said his ruling was also based on Barbour's illegal entry into the residence and his previous abusive relationship with Pittman, the mother of his three children.
Pittman's and Barbour's children, ranging in ages from 5 to 2, were staying at their grandmother's Chambersburg home at the time of the shooting.
The night before the shooting, police said Barbour showed up at the home shouting threats and "expletives" with the intent to cause some sort of harm.
Nelson said Pittman had the gun "because they were afraid of him."
At the time, Barbour was living with his sister on Edgar Avenue and at the Franklin County Shelter for the Homeless.
Pittman had filed a protection from abuse order against Barbour but dropped the order three weeks before, court records show, "because she thinks he has made a change in his life."
Barbour had indicated to Pittman that he had reformed, that his stay in jail had changed him and that he was "remorseful," according to Nelson.
Pittman also asked the Franklin County district attorney's office to drop the indirect criminal contempt, harassment and assault charges against Barbour.
The protection from abuse order was filed in August after Barbour was charged with harassment and simple assault, according to Franklin County Court records.
The order prohibited Barbour from having contact with Pittman and from harassing her and her relatives.
Barbour was sentenced to 90 days in Franklin County Prison in October 1996 for violating the protection order three times, court records said.
Pittman filed the first protection from abuse order against Barbour in May 1993, alleging verbal and physical abuse of her and the children, court records said.
Listed on the order are several incidents of alleged physical abuse dating back to November 1992.
Pittman and Barbour, who attended Chambersburg Senior High School, met about six years ago but never married, Pittman's mother said. She said the couple had problems "off and on" throughout their relationship