SAN JOSE -- It was the jury's turn to do what it wanted.

More than four years after Jae Williams killed a fellow Santa Teresa High School classmate -- merely because he wanted to, he told police -- jurors took only one day to find him guilty after a monthlong trial.

And not just guilty of helping a friend carry out a manipulative and sadistic plan to butcher 15-year-old Michael Russell; guilty of actually wielding the chef's knife they took from his friend's mother's kitchen, an enhancement that consigns him to spend the rest of his life behind bars for a brutal act that shocked the community.

"It's a good day for the people," District Attorney Jeff Rosen said outside the courthouse.

Whatever momentary thrill Williams confessed he got out of killing Michael at age 15 was supplanted Wednesday by the sting of the verdict. Williams, now 19, bent his head and wept so hard his tears fell on the glass-topped table. His sentencing was set for July 11.

Williams was a sophomore on Nov. 10, 2009, when prosecutors say he and alleged accomplice Randy Thompson strolled over to Michael's house carrying two knives, lured him outside with the prospect of smoking marijuana, then tackled and repeatedly stabbed and possibly choked the boy to death in his own south San Jose backyard.

Williams was tried as an adult. Thompson, who was 16 at the time, will stand trial, also as an adult, after Williams' case. He too faces a life sentence if he is convicted. Jury selection begins in July. Santa Clara County Superior Court Judge Arthur Bocanegra decided to hold consecutive trials because Thompson and Williams implicated each other in police interviews.

Michael's grief-stricken mother, aunts and other relatives came to court every day and have vowed to also faithfully attend Thompson's trial. On Wednesday, they said they have been advised not to comment on the verdict because Thompson's trial is still pending. But they looked relieved. Outside court, one of Michael's relatives quietly said, "Yeah!"

More than 70 people packed the courtroom, including Rosen and the San Jose police detectives who elicited Williams' confession. Williams' family also attended, dabbing their eyes as the verdict was announced.

In a videotaped confession two days after Michael was killed, Williams told the detectives that he and Thompson dabbled in Satanism and had already beaten a cat to death, adding, "I guess I just finally wanted to kill somebody. I had my chance and took it."

Pressed at least six times by the officers about why he would kill a friend, he said, "It might not be a very good reason, but there is a reason," Williams said. "'Cause we wanted to."

During the interview, Williams gave several versions of his role in the brutal stabbing, ranging from merely watching Thompson, to tackling Michael and pinning him down, to slitting his throat to make sure he was dead. Prosecutor Valerie McGuire told the eight men and four women on the jury Monday in her closing argument that Williams was guilty of premeditated murder even if they believed he only aided and abetted Thompson by holding Michael down.

Jurors declined to comment Wednesday, but by finding him guilty of the knife enhancement, they indicated they believed he fully participated.

The evidence against Williams included a chef's knife taken from Thompson's kitchen and stained with Michael's blood, and a sweatshirt containing Williams' DNA and Michael's blood. Williams told police he tossed the weapon, sweatshirt and another blood-stained jacket with Thompson's DNA in a Dumpster; police found them nearby in the bushes.

Lead defense attorney Lewis Romero contended that Thompson and his older brother carried out the killing and that the Thompson family engaged in a cover-up. He also brought in experts who testified that Williams gave a false confession, in part because he was sleep-deprived from being kept overnight in a holding cell at the station.

"It was a difficult case," said defense attorney Paul Alaga outside the courtroom, noting the challenge in overcoming Williams' confession. "There were problems with the way the statement was extracted. I don't think what he said happened was what happened."

McGuire said Michael was "butchered" by the two defendants, who stabbed him at least 12 times. The causes of death were stabbing and asphyxiation, either from a stab wound or from being choked. The attack lasted six minutes, according to Williams' confession.

Michael was found later that night by his uncle, whose plaintive attempts to revive him were heard by the jury via a 911 recording.

In his closing argument Monday, Romero tried to persuade jurors to acquit Williams, saying at least twice that he is personally opposed to "locking children up in adult cages for the rest of their lives." Jurors are barred from factoring potential sentences in their decision, and Judge Bocanegra ordered them to discount the remark -- advice they apparently heeded.

Staff writer Robert Salonga contributed to this report. Contact Tracey Kaplan at 408-278-3482. Follow her at Twitter.com/tkaplanreport.