Tyrelle Harris, 19, said he tried to break up a dispute between Karl Gallagher, 31, and another man as they waited for pizza at West Cross Shopping Centre, in Smethwick, on April 26.
More from the trial:
But Harris, who denies murder, said Mr Gallagher quickly turned aggressive, Wolverhampton Crown Court was told.
Harris, of Helvellyn Way, in West Bromwich, denies murder on the grounds of self-defence but has already pleaded guilty to having a bladed article.
Both men arrived at the shopping centre in two separate groups when Mr Gallagher asked one Harris’ associates “what are you looking at”, jurors were told.
Harris said he then tried to stop the situation from escalating but Mr Gallagher, a father and personal trainer, got in his “personal space”, he told the court.
Harris said he reacted by pushing Mr Gallagher which caused him to become “even more aggressive”.
“He said ‘are you stupid? I will shoot you for that’,” said Harris.
The trial continues.