A one-time gang leader will serve at least 30 years in state prison for shooting a former ally in the back, paralyzing him and leaving him to die, all because the victim tried to renounce gang life, Suffolk County District Attorney Daniel F. Conley said.

Suffolk Superior Court Judge Frank Gaziano sentenced DONALD RAY WILLIAMS (D.O.B. 10/4/84) of Dorchester to 19 to 20 years in prison for the attempted murder of a 20-year-old man who had once been his best friend, an additional 11- to 15- year term for inflicting injuries that will require the man to use a wheelchair for the rest of his life, and concurrent house of correction terms for unlawfully carrying the loaded firearm he used in the attack.

Williams will begin serving the cumulative 35-year sentence only after he completes the eight- to nine-year term he is currently serving for an unrelated incident in which he shot a different unarmed man in the back. He committed this most recent offense while free on bail for the earlier one.

“Donald Ray Williams deserves every day of this sentence,” Conley said. “Boston will be a safer place without him. Step by step, witness by witness, the evidence proved that he targeted a man who wanted to give up the violence associated with gang life, tried to kill him, and then tried to keep him from testifying. His actions were cowardly and unconscionable, and he’ll have more than three decades to reflect on them.”

Testimony began April 11. Lawyers gave closing arguments on April 20. Jurors deliberated for about a day and a half before convicting him of all indicted charges.

Prior to sentencing, prosecutors read a statement from the victim, who faced down his attacker during two days of direct and cross examination.

“I loved you like a brother,” the statement read. “It didn’t have to be like this. The one time I really tried to do something good, it was bad for me but in a good way. It showed me that everyone I thought was my friend really wasn’t, but now I know who’s really my friends.”

Assistant District Attorneys Gretchen Lundgren and Melissa Brooks introduced evidence and testimony proving that the victim, then 20, had been a member of the Greenwood Street Packers, the street gang Williams led. On the night of Nov. 23, 2007, the victim testified, he met with members of that gang and told them he was giving up gang life.

The victim returned the handgun Williams had given him and bought his friends a meal as he prepared to give up the life they had shared. In the early morning hours of Nov. 24, they drove him to his girlfriend’s home on Torrey Street in a two-car caravan.

As the unarmed man walked toward that residence, the evidence showed, Williams got out of one vehicle, walked up behind him, and fired three shots into his back. The victim fell to the ground and turned to face the man who had once been his best friend.

“Bitch-ass [expletive],” Williams said, pulling the trigger again and shooting the man in the face. “That’s what you get.”

Grievously wounded and unable to use his legs because his spinal cord had been damaged, the victim saw his cell phone where it had fallen a few feet away. After Williams and the other men drove away, the victim crawled to the phone, replaced the battery that had fallen out, and called his girlfriend’s sister.

The victim told the young woman that he had been shot, that he was about to die, and that she should tell his brother that “Mann” shot him. No one else, he testified, would have believed her because he and Williams had been so close.

Emergency medical personnel raced him to Boston Medical Center, where he underwent emergency surgery and remained in an induced coma for weeks. He identified Williams as his shooter to first responders, to police at the hospital, to the Suffolk County Grand Jury, and, last week, to a packed courtroom.

No other members of the Greenwood Street Packers testified. Two who were subpoenaed refused to take the stand and the court ruled that both had Fifth Amendment privileges.

Jennifer Stott was the DA’s assigned victim-witness advocate.