A teenager has been convicted of the murder of a disabled man who was kicked to death by a gang "for sport".
Brent Martin, 23, who had learning difficulties, died in hospital after being found in a pool of blood on Sunderland's Town End Farm estate.
The 17-year-old, who cannot be named, and two others beat their victim in a £5 bet over who could knock him out, Newcastle Crown Court heard.
William Hughes, 21, and a youth, aged 16, had already admitted murder.
The teenagers, who trained as boxers, repeatedly punched, kicked, stamped on and head-butted their victim.
A post-mortem examination revealed Mr Martin died from a massive head injury and had suffered at least 18 separate blows to the head and neck.
During his trial, the court heard that the 17-year-old defendant later told witnesses: "I am not going down for a muppet."
The court heard how Mr Martin was chased for a mile and a half across two estates in Sunderland and subjected to a series of brutal assaults.
Finally he was stripped of his trousers and pants and left dying in a pool of blood next to a parked car in Baxter Road, Town End Farm, as his attackers posed for photos.
Toby Hedworth QC, prosecuting, said the gang acted like "a pack of animals" during the attacks on their vulnerable victim, who had spent nine years of his life in psychiatric hospitals."
Judge John Mitford said he had asked Northumbria Police to supply figures for the number of murders in the force area over the past 10 years.
He said: "I'm particularly interested in the growing trend where defendants are getting younger and younger."
Mr Hedworth added: "As the attack went on, its nature, ferocity and perseverance made it quite clear they were not happy until he was dead."
William Hughes had already admitted murder
Mr Martin's mother Brenda said her son was dead because he did not fight back.
She said: "He had such an impish smile and thought everyone was his friend.
"He spent eight years in hospital and was very short-sighted and had a lot to learn and cope with, with his disabilities.
"He was very pleased to come out of hospital and see the lads - but things had changed.
"He was a gentle giant and they thought he was a simpleton.
"He was vulnerable and they thought he was soft - he did not retaliate - if he had he would be here today."
The jury of eight men and four women took four hours to convict the teenager, who sat emotionless as he was found guilty.
Judge John Milford QC warned the defendant and his two co-accused, who were not in court, they faced a mandatory life sentence.
Sentencing will take place at a later date.
The mother of the 16-year-old has been charged with attempting to pervert the course of justice by washing her son's bloodstained clothes.
No date has yet been set for her trial.