Alain Paul de Cock, sobbing through most of an hour-long appearance before a Fairfax County judge, pleaded guilty yesterday to murdering his mother and father in the family's McLean home last December.

"I was hurt. I wanted to hurt her," the 22-year-old de Cock said, detailing the emotional tumult that he said led to the shooting death of his mother, Simone Irene de Cock, 50. That slaying was followed minutes later by the shooting of his father, Romain Paul de Cock, 52, a World Bank loan officer.

Defense attorney Robert Whitestone told the judge that de Cock "snapped" under mounting family pressures the day of the shootings. He felt rejected by his parents and harassed by his mother's nagging, Whitestone said, adding, "There was an explosion."

Chief Circuit Court Judge Barnard Jennings accepted the guilty pleas to two first-degree murder charges and two firearms charges in connection with the killings and scheduled sentencing for March 30.

De Cock could be sentenced to 20 years to life in prison for each of the murder convictions. State law mandates a two-year prison term for the first firearms charge and a four-year sentence for the second. De Cock could be eligible for parole in 20 years if he is given two life sentences, said Commonwealth's Attorney Robert F. Horan Jr.

De Cock, frequently sobbing uncontrollably, recounted details of his family life and the events surrounding the death of his parents. He said he and his parents, especially his mother, argued constantly and that the rift between them began about six years earlier when his older sister became ill with cancer. It intensified when she died in July 1981, he said.

Last Dec. 13, he said, when he returned home from a shopping trip he received a stream of admonishments from his mother, who was upset that he had wrecked her car several days earlier.

De Cock said he was suffering from a piercing headache, so he ignored his mother and went upstairs to find an aspirin as she continued to scream at him from downstairs.

Finally, de Cock testified, his eyes shut and his face red from crying: "She told me that she wished that my sister were alive. She said that my sister was a better child."

It was then, psychologist Joseph R. Sanders testified, that de Cock "flipped into an altered state" as result of prolonged, pent-up anger at his parents.

De Cock testified that he was feeling rejected and desolate when he took his loaded .22 rifle from his bedroom closet and walked down the hall into his mother's bedroom.

"I wanted to do something, but I didn't know what," he said. "I wanted to shoot myself so I wouldn't have any more problems. My mother walked in. I shot her."

He said that as he started to leave the house, he heard his father come in and he aimed the rifle at him, firing from the top of the stairs and hitting his father in the back of the head.

When prosecutor Horan attempted to show de Cock a photograph of his father's body sprawled at the foot of the stairs, de Cock turned to the judge and said, weeping openly, "Judge, I don't want to see that."

An uncle from Nice, France, seated on the front row of the courtroom with several other European relatives and a World Bank official, burst into tears.

Earlier in the hearing, de Cock buried his head in his hands when attorneys asked him to identify the rifle used in the slayings.

Psychologist Sanders testified that de Cock, who speaks five languages fluently and lived in Rome, Nicaragua, the Dominican Republic and his parents' native Belgium before the family moved to Fairfax County in 1979, is in the upper 10 percent of the nation in intelligence. He also described de Cock as emotionally stunted and suffering from a "character disorder."

"It is clear this defendant shot both parents," Horan said in closing arguments. "He shot his mother in the eye. . . then shot his father so he could escape and create an alibi." De Cock was arrested three days after the slayings after police found his rifle hidden in a crawl space.