A Pico Rivera man convicted of fatally shooting a 19-year-old gang member and burying the body in his backyard has had his murder conviction reduced to voluntary manslaughter, authorities said.

Robert Charles Redd, 53, was sentenced Thursday to 10 years in prison rather than the mandatory 40-years-to-life term he would have received for second-degree murder with the use of a firearm, said Los Angeles County Deputy Dist. Atty. Brandon K. Wong.

Wong said Superior Court Judge Raul A. Sahagun reduced the conviction, saying he disagreed with the murder verdict that Norwalk jurors reached in April and believed that Redd shot Joseph Rubalcaba while in fear for his life, but that his fear was unreasonable.

While judges have the authority to reduce or throw out a jury's verdict under certain circumstances, such decisions are rare in murder cases.

"I’m disappointed, obviously, and I disagree with the judge’s ruling, but it is within his discretion," Wong said.

The prosecutor said the maximum sentence Redd could have received for voluntary manslaughter with a gun was 21 years in prison. The district attorney's office has yet to decide whether to appeal, Wong said.

Redd's attorney could not be reached for comment.

Melinda Rodriguez, the victim's aunt, said her family had been shocked when the judge announced he did not agree with the unanimous verdicts of 12 jurors, who considered but rejected manslaughter in the case.

"The shock is wearing off and the pain is coming in," Rodriguez said.

She described her nephew as a smart and inquisitive boy who ran into legal trouble after beginning to associate with gang members. She said he embraced Christianity a week before his death and was trying to turn his life around.

Wong said the defense argued that Rubalcaba was a violent gang member and presented evidence that he had committed a robbery using a firearm two weeks before he was killed. He had previously been caught in possession of a gun and was shown in one gang photograph holding a pistol to his forehead. 

A gang expert testified that Rubalcaba's tattoos of "Rivera" over one eyebrow and "kills" over the other would likely have been added after he had killed someone, the prosecutor said.

Redd told sheriff's detectives after the slaying that he had opened fire because Rubalcaba had a .380-caliber handgun. Redd claimed during the interview that he had thrown the victim's pistol into the San Gabriel River, but authorities were unable to find the weapon despite conducting a thorough search, Wong said.

The prosecutor said he argued during the trial that Redd had concocted the story about Rubalcaba having a gun and that the shooting was over a drug dispute. He noted Redd told detectives  he had allowed Rubalcaba to live at his Pico Rivera home in exchange for methamphetamine but that Rubalcaba began to stay at the home more and more without supplying the promised drugs.

After the shooting, Wong said, Redd dumped the victim's body upside down in a recycling trash can and left it in his bedroom. The body, however, began to smell after some time, so Redd buried it in the back yard, the prosecutor said.

"It's like he was taking out the trash -- no respect for the dead body," Wong said.

Rubalcaba's relatives discovered his buried body after growing concerned by his disappearance and conducting a search for him.

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jack.leonard@latimes.com

Twitter: @jackfleonard