Authorities say it was a case of rage, apparently sparked by a neighbor's barking dog running loose.
25-year-old William Satterfield of Bethesda is charged with fatally shooting his neighbor, the dog owner.
25-year-old Kevin Smith was shot in the forehead in the early hours of Oct. 7.
He was flown to a Pittsburgh hospital, where he died about a week later.
Today his girlfriend, Grace Wineman, sobbed on the witness stand as she recalled the shooting.
She said Satterfield came to their home, standing outside, screaming obscenities at Kevin Smith through the front window.
Wineman said Smith started to shout back at Satterfield, then two shots rang out, and Smith fell off his perch on the top of the couch, falling through the front window, and onto a barbecue grill on the ground below.
Defense attorneys for Satterfield contend that Smith was outside slapping Satterfield, and that he jogged Satterfield's arm, prompting him to fire the gun accidentally.
They say the death was not a murder, but a tragic accident.
Prosecutor Chris Berhalter told jurors they will hear the 911 tapes of Satterfield, insisting that he "had to shoot a man" and that he is a member of the 82nd Airborne Division.
Smith and Wineman's dog, Rex, had apparently been running loose earlier in the evening, although Wineman says Rex was in the house with them when the shooting occurred.