The sharp sound of the defendant slamming his hands into the wooden door ricocheted around the courtroom, an echo as raw as the killing that triggered it.

Renaldo A. Mathis Sr. angrily stormed from the District courtroom yesterday to begin serving a maximum sentence of 20 years to life in prison for stomping police informant Eric Butera to death in December 1997.

D.C. Superior Court Judge Nan R. Shuker described Butera as "a young man who had a right to live."

An admitted cocaine user, Butera was working undercover that night, helping police follow a tip about the 1997 triple slayings at a Starbucks coffee shop.

"If Mr. Butera had been on the FBI's most-wanted list, he still had a right to live," Shuker said. "It doesn't matter if he was better . . . or not better."

Before the attack, Butera told police he had heard talk of the Starbucks slayings at a house where he had bought cocaine. Detectives gave Butera $150 in marked bills and told him to buy cocaine at a house in the 1000 block of Delaware Avenue SW.

People inside the house turned Butera away, Assistant U.S. Attorney Kenneth L. Wainstein told a jury in April. As Butera walked down a nearby alley, Mathis's brother and nephew hit him, stole the marked money and left him on the ground. The attack was not connected to Butera's police work or the Starbucks case.

Mathis then ran up to Butera, held onto a fence post to steady himself and stomped on Butera's head, according to testimony at trial. The attacker then walked away and spent the rest of the night talking idly with friends.

The jury convicted Mathis on a second-degree murder charge, rejecting his lawyer's claim that the other men were the "real killers." Keith Mathis and Robert Walker pleaded guilty to robbery and assault charges.

"The truth is, I have no idea how I'm going to live my life without my son," said Butera's mother, Terry, who has filed a civil lawsuit against District police. "I don't know what in God's name I am going to do if he doesn't come home. My mind understands the reality, but my heart is simply not ready to give up hope."

Mathis, who Shuker said has nine prior convictions, wrote a letter of apology to Terry Butera. He said, "If you can, please try to forgive me.

"On behalf of me and my family, I want to apologize the best way I can for what we did to your son," Mathis wrote from jail on June 18. "I certainly should not have gone near your son, except maybe to help him. I am ashamed and very sorry for my actions."

Mathis bragged about the killing and threatened to kill his own relatives if they snitched on him, Wainstein said. He added that the suspect also promised to "cancel" an elderly neighbor who had talked about the case with reporters.

Mathis will not be eligible for parole until 2016 at the earliest.