Watta had been serving a 10-year sentence for attempted murder in Florida when he was linked to the woman's death through DNA evidence.
A defense attorney presented evidence that convicted Texas serial slayer Henry Lee Lucas confessed to Sharpe's murder in 1986, but Senior Deputy District Attorney Larry Yellin argued that DNA evidence conclusively matched Watta to the crime.
Watta was a registered sex offender in California and has prior convictions for attempted rape, burglary, use of a firearm and attempted murder of a police officer, said Farrah Emami of the Orange County District Attorney's Office.
During the penalty phase of the trial, defense attorneys portrayed Watta as suffering the effects of a childhood that included sexual abuse and abandonment by his mother when he was 6 months old, leading to years of placement in foster homes and other institutions.
Police earlier said Watta had spent much of his adult life in prison, for crimes ranging from burglaries to sexual assaults, kidnapping and firearms violations.
A 1988 conviction in Sacramento for the attempted murder of a police officer, rape and possession of stolen property drew an 11-year sentence, of which he served about six.
According to prosecutors, Watta's prison term in Florida stemmed from a shooting attack and attempt to kill his then-girlfriend.
Prosecutors in Orange County decided to seek the death penalty after learning that Watta was linked, through DNA, to an unsolved rape in Florida.
Jurors were told that the attack took place over a period of five hours, while Sharpe's encounter with Watta may have lasted two or three hours.
She suffered four knife wounds in the throat and would have bled to death, but suffocated because her nose had been broken and, when she fell, it closed off her air passage.
Police speculated earlier that Watta, who had not been out of prison for long at the time of the slaying, saw an open garage door at the neighbor's home in the 1100 block of Coastline Drive and also saw Sharpe and took advantage of a crime of opportunity.
She failed to return home, and when her son went looking for her the following day, on Christmas Eve, he entered the next-door home and found his mother, who had been raped, in a bedroom, wedged between the bed and the wall.
The then-unsolved case was among the first 100 presented for re- examination in March 1998 by the district attorney's TracKRS Project, utilizing the most recent advances in DNA and other science and data technology.
The Seal Beach Police Department was notified on Jan. 26, 2001, that a suspect DNA profile had been created from evidence culled from the crime scene and the results, submitted to a state Department of Justice database on March 1, 2001, came back 20 days later with Watta as the suspect.
He would have been eligible for parole in 2007 for the attempted murder of his girlfriend if not for the DNA match, authorities said earlier.
In 2002, Watta, then 57 years old, was found incompetent to stand trial and was committed to Patton State Hospital for a time, but prosecution was later reinstated.
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