A Prince William County jury decided yesterday that Tony A. Mackall should be executed in Virginia's electric chair for shooting a Woodbridge gas station attendant to death during a holdup last December.

Circuit Judge Herman A. Whisenant Jr. will impose sentence on Dec. 18. Under Virginia law, he can reduce the jury's death sentence. If he does not, Mackall, 23, will become the 33rd person on Virginia's death row, and the third person there because of a crime committed in Northern Virginia.

Mackall, who was convicted on Wednesday of capital murder, robbery and use of a firearm in the commission of a felony, sat solemnly as the court clerk read the jury's decision, which came after two hours of deliberation. Earlier this week, the jury sentenced him to life in prison on the robbery charge and two years on the weapons charge.

Mackall was convicted of fatally shooting Mary E. Dahn, a 31-year-old mother of two, during a holdup Dec. 9 at the Rte. 123 service station where she and her husband worked. Stephen Dahn and the couple's young daughters were at the gas station at the time of the shooting, and April Dahn, 6, was called as a witness in the trial, which began Monday.

She described seeing her mother "bleeding under her head" and said her eyes "were all watery."

Mackall, who at the time of the shooting was an escapee from a D.C. Department of Corrections halfway house, pleaded not guilty to all the charges. He did not testify in his own behalf.

Prince William sheriff's deputies quickly escorted the jurors, some of whom were visibly shaken, from the courthouse through a side exit after yesterday's proceedings, in which the jury said that Mackall presented a danger to society

"It's hard to give anybody the death penalty," a juror said later during a telephone interview. "It's a difficult thing. He is a person."

The juror, who asked not to be identified, said what he remembered most about the case was testimony that Mackall had said the service station "would be an easy place to rob" and that he had been there before the night of the robbery.

That testimony and Mackall's criminal record were key factors in the jury's deliberations, the juror said.

Another juror said the case was too emotionally draining to discuss. "It was hard on all of us," she said.

A third juror said deliberations took a while because "there was a lot of material we had to sift through. We wanted to go through every piece of evidence -- that took time."

No single element persuaded the jury, she said, adding, "We really came to the decision based on all the evidence."

Mackall faces additional charges later this month stemming from a shooting 30 minutes after the gas station robbery.

Prosecutors have said that Mackall crashed his stolen car while fleeing from the gas station, abandoned it and then entered a nearby town house where he shot Michael Keating, sports editor of The Washington Times.

Keating, 28, who was shot twice in the head, testified on Tuesday that he pleaded with his assailant not to shoot.

The jury's decision followed a daylong sentencing hearing on Thursday during which Commonwealth's Attorney Paul B. Ebert portrayed Mackall as having been in and out of trouble all his life, and as someone with little regard for human life.

"He hated women, people in general," Ebert said, referring to the testimony of a psychologist who counseled Mackall as a youth. "He looked at people as 'blobs,' " Ebert said. "No feeling. No heart. No compassion . . . . This man has a violent bent."

Ebert described Mary Dahn as hard-working and "not deserving of her fate."

"The Mrs. Dahns of this world have a right to be heard," he told the jurors. "She speaks to you in a silent way."

Ebert then turned to the Keating shooting, saying: "If that doesn't indicate future danger, I don't know what does."

Mackall's attorney, Ronald Fahy, told the jurors that "Mary Dahn is dead and there is nothing you can do to bring her back. Perhaps, if this was a more perfect world, we could exchange lives.

"Michael Keating was shot. There is nothing you can do to change his pain."

Fahy said Mackall had sought help the day of the shooting, but couldn't find any. That was the story of his life, Fahy said.

The attorney described Mackall as an abused child raised in a fatherless home, and as someone who had been in institutions on and off since he was 9.

"Tony was deprived, not depraved," Fahy said.

Mackall had no dignity, nothing left, except for his life, Fahy said. "I am asking you not to take the last thing."

Outside the Manassas courtroom yesterday, Ebert said that the D.C. Department of Corrections shares responsibility for Dahn's death.

Last November Mackall was serving time at a District halfway house for burglary and unauthorized use of an automobile. On Nov. 26 he failed to return to the facility, corrections officials have said. After his arrest in the Prince William shootings, county and D.C. officials said Mackall had been missing from the halfway house for 11 days before a warrant was issued. The warrant came one day before Dahn's murder.

Ebert said that on the day Dahn was killed, Mackall visited his social worker at the District's Lorton prison complex in Fairfax County, in a stolen car, and was not apprehended because the social worker did not know Mackall was an escapee.

The social worker could not be reached for comment yesterday.

After the jury imposed the death sentence yesterday, Stephen Dahn said he had anticipated it. "I was expecting it," he said. "I just had a funny feeling." He said Mackall's execution "will never bring back Beth, but it is what he deserves."

Jane Graham, Mary Dahn's aunt, agreed, saying: "I am glad that he is going to have to think about somebody taking his life."

Virginia has 32 people on death row, two of whom were sentenced for crimes committed in Northern Virginia: Timothy Dale Bunch, convicted in Prince William County in 1982 of shooting and hanging a Dale City woman, and Wilbert Lee Evans, convicted in 1984 of killing a sheriff's deputy in Alexandria.

Since Gov. Gerald L. Baliles took office in January 1986, two men have been executed in Virginia, one of them for a crime committed in Fairfax County.

Richard Whitley, who killed his 63-year-old Fairfax County neighbor in July 1980, was electrocuted in the state penitentiary in Richmond in July.Staff writer Peter Pae contributed to this report.