Man Guilty in Slaying of Nurse : Crime: Ventura County jury takes five hours to convict Mark Thornton of the murder of Kellie O’Sullivan. He could face the death penalty.

A Ventura County jury, deliberating for less than five hours Monday, convicted a 20-year-old Thousand Oaks man of murder and a special circumstance that could send him to the gas chamber for last year’s slaying of Westlake nurse Kellie O’Sullivan.

Mark Scott Thornton, whom prosecutors branded a cold-blooded killer but defense attorneys called an immature and impulsive young man, sat stoically as Superior Court Judge Charles R. McGrath read guilty verdicts to the 13 counts he faced.

“He’s very upset about the verdict as far as the special circumstance,” said Deputy Public Defender Susan R. Olson, adding that Thornton broke down and sobbed as soon as he left the courtroom. “He’s very frightened about what he’s facing.”

Prosecutors said they were pleased with the jury’s findings and will continue to push for the maximum punishment when Thornton’s penalty hearing begins in about three weeks.

“There’s only two choices, and we will be requesting a death sentence,” Deputy Dist. Atty. Peter D. Kossoris said.

The most serious charge against Thornton involved the Sept. 14, 1993, carjacking and killing of O’Sullivan, the mother of a young son. Prosecutors believe he killed her for her car, which he later used to abduct his former girlfriend.

Prosecutors told the jury that Thornton kidnaped O’Sullivan in broad daylight in front of a Thousand Oaks pet store, forced her to a desolate area in the Santa Monica Mountains and shot her once in the chest and twice in the back.

O’Sullivan’s badly decomposed body, still clad in her nurse’s uniform, was found 12 days later by two volunteers who had joined hundreds of others in a massive demonstration of community unity to search for the missing woman.

Thornton had been arrested seven days earlier in Reno, where he was in possession of O’Sullivan’s Ford Explorer vehicle and a .38-caliber snub-nosed revolver, which later was proven to be the murder weapon.

Despite that evidence, the clean-scrubbed, soft-spoken defendant maintained his innocence after his arrest in an interview with police and during a taped jail visit with his 68-year-old grandmother.

Thornton never took the witness stand, and his attorneys did not admit he was the killer until closing arguments last week. Still, they implored jurors to find him guilty of only first-degree murder and not the special circumstance that could send him to his death.

The defense attorneys told the jury that Thornton is immature and speculated that he probably killed O’Sullivan in a moment of panic, perhaps as she lunged for his gun.

Prosecutors called such speculation absurd and urged the jury to return a verdict that would make Thornton eligible for the gas chamber. After Monday’s verdict was read, jurors remained bound by an admonition not to speak about the case until after they make a recommendation for his penalty--life in prison or death.

But clearly, everyone involved in the case was surprised by how quickly the jury came back with its decision of guilt. The jury began deliberating about 9:30 Monday morning. About 4 p.m., the foreman told the bailiff that decisions had been reached on all counts.

Prosecutor Kossoris acknowledged that the jury wasted little time reaching a decision, but said he did not want to read too much into it at this stage in the proceedings. “From trying cases for 28 years, I never can even predict myself how long they will be out,” he said.

Defense attorneys also were reluctant to comment on the quick verdict.

As workers from the public defender’s and district attorney’s officers crowded into the courtroom to watch, McGrath questioned the jury on each guilty verdict. In addition to murder, kidnaping and robbery, the jurors returned guilty verdicts on forgery, grand theft and non-sufficient funds check charges--all crimes committed to aid Thornton’s escape from the county after he stole the nurse’s vehicle, prosecutors have contended.

Thornton was also convicted of using O’Sullivan’s vehicle to kidnap his former girlfriend and of shooting at the girl’s mother during that abduction.

During the seven-week trial, the prosecution said the defendant needed O’Sullivan’s vehicle to abduct 16-year-old Stephanie Campbell and leave town. He wanted to leave town because juvenile authorities had issued a warrant for his arrest for a probation violation and because he had cashed a series of bad checks, prosecutors argued. Thornton also wanted to take Campbell with him because she had recently broken off their relationship, prosecutors said.

Campbell was a key witness against Thornton, testifying that during the five days between her abduction and Thornton’s arrest, she was afraid for her life. Thornton, she said, had threatened to kill her if she tried to escape as they drove around Northern California and then to a Reno casino. There, she notified a security guard that she had been abducted.

In the next phase, Thornton’s attorneys said they plan to show jurors that Thornton had suffered a terrible childhood and hope to elicit some sympathy. “It’s obviously a very difficult task we’re facing now to try and get together this young man’s life to show what led up to Sept. 14, 1993,” Olson told reporters. “Anyone who sat through the first phase of the trial knows that there was a dysfunction in his family and at school.

“He has a long history of turmoil in his family. He never knew his biological father. . . . He had three stepfathers. . . . We’re trying to save this young man’s life. He doesn’t deserve to die.”

Staff writer Carlos Lozano and correspondent J. E. Mitchell contributed to this report.