A gang of three who punched, kicked and stamped a disabled man to death in what a judge called a sadistic attack were sentenced to life in prison yesterday.

Brent Martin, 23, was chased around the housing estate where he lived in August last year by the three after they placed a £5 bet on who could knock him out first.

Mr Martin, from Sunderland, who suffered from learning difficulties, had just been released from a psychiatric hospital.

The gang ringleader, William Hughes, 22, and Marcus Miller, 16, pleaded guilty to murder. Stephen Bonallie, 17, denied the charge but was sentenced yesterday to at least 18 years in jail. Hughes will serve a minimum of 22 years, while Miller must serve at least 15 years.

Sentencing the men at Newcastle Crown Court yesterday, Judge John Milford described their crime as “sadistic”. He said their victim was a “gentle and caring person” with a long history of mental illness, and was simply looking for friends.

“Sadly, he made poor choices,” the judge said.

He told the three men: “You seemed in a state of some elation and went on with your lives. He died from his head injuries.”

Toby Hedworth, for the prosecution, said: “They behaved like a pack of animals as they repeatedly punched, head-butted, kicked, stamped on Brent Martin, who never lifted an aggressive finger towards one of them, and they did so until he was dead. They did so for their own sport.”

Miller and Bonallie were both trained boxers, the court was told.

One 13-year-old witness said: “I thought once they nutted him out they were going to leave him, but they never. They carried on hitting him and hitting him.”

The court was told that even as he was being beaten, Mr Martin kept trying to shake hands with his attackers.

But after being slammed against a car so hard it caused a dent, and then a “volley kick” from Hughes, Mr Martin suffered fatal brain damage.

Ben Nolan, defending Miller, the youngest of the gang, said the teenager had an IQ of 79. He blamed cheap and readily available alcohol for the attack. “Alcohol is readily available at all times of day and night at a ridiculously low cost,” he said.

When the verdicts were announced, Mr Martin’s family cheered from the gallery. Mr Martin’s mother, Brenda Martin, wearing a white T-shirt with her son’s picture above the word “murdered”, later said outside the courthouse: “I’m just very pleased with the verdict. The judge was brilliant. The Government has to listen to murder victims’ families.”

After the trial, Judge Milford asked Northumbria Police to compile figures on the number of homicides committed by people under 21 in the past five years, compared with a decade ago.

Mr Hedworth said that between 1992 and 1997 in the Northumbria Police area there were 32 homicides committed by people under 21, and four by under-17s.

Between 2002 and 2007, he said, there were 63 homicides by under-21s, with 13 killings by under-17s in the Northumbria Police area.

The judge said the figures for under-21 killings had risen by almost 100 per cent and by under-17s by 300 per cent. He said: “These are extremely concerning statistics, even if the figures for this particular region are well below figures in other areas and detection rates here are higher.”

Judge Milford said he considered setting deterrent sentences on the three because of the sadistic nature of the attack, but decided to stick to the guidelines set out in law.

The judge said: “I would hope the minimum terms imposed upon young offenders are sufficiently long to spell out to young people the consequences of committing murder.

“If I am wrong then perhaps the 12-year starting point for those under 18 needs to be looked at again by Parliament when the murder is, as in this case, particularly serious.”

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