A man accused of murdering an Arsenal fan on the London Underground had previously been convicted for a stabbing, a court heard.
Alex Lanning has admitted fatally stabbing Tashan Daniel in the heart at Hillingdon Tube station last September.
Jurors at the Old Bailey heard the 22-year-old was convicted in 2016 of stabbing a man in Brighton during a row over a Class A drug deal.
Both he and co-defendant Jonathan Camille deny murdering Mr Daniel.
Jurors heard Mr Lanning was 18 when he pleaded guilty for drugs offences and unlawfully wounding William Goodfield.
Prosecutor Will Martin told the Old Bailey Mr Lanning had met Mr Goodfield on 5 March, 2016, to sell Class A drugs.
He said Mr Goodfield did not have money, so produced a knife and attempted to rob Mr Lanning.
The knife “ended up in Mr Lanning’s hands”, who “deliberately stabbed Mr Goodfield, including to the torso”.
Mr Lanning had initially acted in self-defence, “but subsequently his actions went beyond lawful self-defence,” Mr Martin said.
Before finishing his evidence, Mr Lanning, of Uxbridge, apologised to Mr Daniel’s family, who were in the courtroom.
“I am genuinely, genuinely sorry to the Daniel family from the actions I’ve caused,” Mr Lanning said.
“I know I’ll have to live with this for the rest of my life and my words are not enough.”
After stabbing Mr Daniel on 24 September, Mr Lanning said, he had “slept rough” and arranged to meet Mr Camille on 4 October to “discuss what to do”.
“I had mixed emotions. I didn’t really know what to do,” he said.
“I was confused, scared and worried and I was partly about my family and the family of Tashan Daniel.”
The pair met in central London “for a couple of hours” but ended up getting arrested on Cheapside in the City of London.
Mr Lanning also added that his co-defendant acted as a “peacemaker” when a fight broke out on the platform of Hillingdon station between the two defendants and Mr Daniel and his friend Treyone Campbell.
“He asked him what was going on and telling him to chill, from what I recall,” Mr Lanning said.
The trial continues.