SAN DIEGO -- A gunman who was a teenager when he shot a 20-year-old man in the back during a drug deal gone bad was sentenced Tuesday to 50 years to life in state prison.

Tyierre Christian Perry, now 20, was convicted Feb. 4 of first-degree murder, robbery and attempted robbery.

Haremi Watts told Judge Leo Valentine Jr. at the sentencing hearing that her son Spencer was killed on April 22, 2006, three days after he turned 20.

Spencer Watts, his twin brother and their mother celebrated their April birthdays each year by dining on sushi, as part of their Japanese heritage, the mother said.

"(Now), our Aprils are ruined," she said.

The mother said the victim spent a lot of time writing music with his twin, who is now "unable to do anything because he lost his brother."

The victim's father urged the judge to take something from Perry because "he took something from me."

"I can't get it back," James Watts told the judge. "We don't live in a world where you just take what you want. He (Perry) shouldn't be able to walk the streets again. He's less than a person."

The victim's aunt, Janice Scott, said Perry "was a coward" to shoot her nephew in the back."

"To have an animal walking the street like that ... it's unconscionable," Scott told the judge.

Prosecutor Clay Biddle said Perry, then 16, shot Watts when he wouldn't cooperate with the defendant and another man who wanted to rob the victim and a friend during what had been planned as a sale of ecstasy in Watts' BMW in the College Grove shopping center.

The second assailant remains at large, the prosecutor said.

Watts didn't appear to know the robbers, but Perry said he was an Oceanside gang member, the prosecutor said.

The victim was about to back his car out of a parking spot near a Wal-Mart when Perry pulled out a gun and ordered him to stop and give him everything out of his pockets, Biddle told the jury.

Watts' 15-year-old friend, Keenan Wheeler, complied with the demand, but Watts balked, the prosecutor said.

A man with Perry got out of the car and told him to shoot the victim, which he did, Biddle said.

In November 2006, a friend of Perry's told Watts' former girlfriend that the defendant shot the victim but didn't intend to kill him, the prosecutor told the jury.

After his arrest in December 2007, Perry told detectives he "didn't want to screw himself" and said the shooting was accidental, Biddle said.

Defense attorney Bernard Skomal argued that the evidence left a reasonable doubt as to whether Perry was the killer.

One of Perry's fingerprints, found on the back door of the victim's car, only proves that the defendant touched the car at some unknown time, Skomal argued.

Even though the prosecution claims Watts did not know his killer, the evidence shows that the murderer knew the victim, the defense attorney told the jury.

Wheeler -- the only witness to the murder -- did not identity Perry as the assailant, Skomal said.

Valentine said that although Watts and Wheeler were engaged in criminal activity, Wheeler didn't expect to be robbed and didn't sign up to see his friend murdered.

"You've done something here that you can't undo," the judge told the defendant, who declined to say anything before he was sentenced.