A man told police he was "very sorry" he stabbed his cousin to death, Belfast Crown Court heard on Tuesday.
However, a Crown QC told the jury that 21-year-old Gavin Maguire had intended to kill Brendan McKay, also 21.
His claims came as he opened the case against Mr Maguire in relation Mr McKay's death on 5 February last year.
He said the cousins watched a football match at a hotel and then went to Mr Maguire's girlfriend's house in west Belfast where the killing happened.
Despite the pair having "had a lot of drink", the lawyer, Ken McMahon, told the jury there was "no sign of any bad feeling between them" but added that Mr McKay had been "teasing" his cousin.
Mr Maguire's girlfriend, Orla McCormick was in her son's room watching a DVD with him while Mr Mckay and Mr Maguire were in the living room of the house at Oak Villa, just off the Springfield Road, but she could hear the pair bickering despite shouting at them to stop.
"She went back to the bedroom after that and she was crying at this stage," said Mr McMahon, when she heard Mr Maguire "scream" her name "and she ran in and saw him holding up the deceased who had a look of shock on his face and he then collapsed".
As Miss McCormick called for an ambulance, Mr Maguire craddled his dying cousin's head but turned and said to her: "Orla I don't think he's breathing."
Mr McMahon said that despite mouth-to-mouth resuscitation and emergency surgery at the Royal Victoria Hospital, Mr McKay died from his injuries.
The case continues at Belfast Crown Court.