Larry Gill, who is accused of killing a state trooper and an Army sergeant when they attempted to intervened in a domestic argument, yelled to his wife, "please forgive me" before he started shooting, his brother-in-law testified yesterday.

Gregory Jividen, a 27-year-old electrician, took the witness stand on the opening day of Gill's murder trial in a Fairfax County Circuit courtroom that was packed with spectators, including uniformed law enforcement officers.

Gill, charged with two counts of first-degree murder, the malicious wounding of Jividen and the attempted murder of an off-duty police officer, has pleaded not guilty by reason of insanity. If convicted, he faces a maximum sentence of two life terms plus 54 years, said Commonwealth Attorney Robert F. Horan, who is prosecuting the case.

Gill's attorney, Blair Howard, said in his opening statement that he did not dispute the physical acts of that evening, but "what this trial is all about is the state of Gill's mind on January 15, 1987."

Howard said he would try to prove that Gill, a 32-year-old carpenter, acted on "irresistible impulse" and could not control his actions to keep them within the confines of the law.

Jividen, who had been living with his sister and brother-in-law in a Lorton town house for five months, told the jury that he heard the two arguing over money and household chores. He said that when he heard his name mentioned, he ran to the bedroom where the two were fighting and told Gill to "pick on someone your own size."

He said his sister Pam blocked him from entering the room and the two packed their bags and left the house. Jividen said that while standing in the parking lot, he looked up to a second-story window and saw a shotgun pointed at him.

He said Gill began firing, and asked his wife for forgiveness. He said he was shot in the back, and his brother-in-law opened fire again moments later, killing two neighbors, trooper Alexander Cochran and Sgt. Dennis Kies, who ran to help after hearing the shots and his sister's screams.

In response to a motion by the defense, Judge J. Howe Brown ruled that the prosecution could not mention the occupations of the two neighbors. Brown said that the victims' occupations were not relevant to the incident and mention of them would be inflammatory.

The trial is scheduled to resume today.