Courtney Eaton, 17, was a national mini-moto champion
A teenager who pulled out a knife during a confrontation in Salford did not realise he had stabbed someone in the ensuing struggle, a court heard.
Stuart Fagan, 18, told Manchester Crown Court he was carrying a knife for self defence in Eccles last year when he got into an argument with four youths.
National mini-moto champion Courtney Eaton, 17, suffered four stab wounds in the incident and died during surgery.
Mr Fagan, of James Henry Avenue, Ordsall, Salford, denies murder.
Giving evidence on Monday, Mr Fagan said he drank nearly half a bottle of brandy before going out to buy cigarettes.
He was walking to a Texaco garage with his 14-year-old girlfriend, her two female friends, and his 13-year-old brother when he was confronted by the youths.
Mr Fagan told the court one of the four made a derogatory comment about his brother's weight before three of them squared up to him.
"I pulled my knife out to try and scare them away. I thought if they saw a knife they would have run off and just leave us," he said.
One of the youths then grabbed his collar and pushed him to the ground as the others said "get him", Mr Fagan told the court.
Chris Sutton-Mattocks, defending, asked: "Were you aware of having stabbed someone?"
He replied: "At first I thought I might have but he just kept coming at me so I didn't think anything of it."
Mr Fagan has admitted carrying the weapon for self protection and has already pleaded guilty to carrying a knife in a public place.
The trial continues.