The woman, who gave evidence behind a screen, told the jury the man stabbing David Headlam didn't seem to care she could see.
The 18-year-old son of Bishop Alonzo Headlam was cornered into the witness' garden on Drayton Road in London at the end of June.
"He was very, very angry," she told the Old Bailey.
She said she was in her living room when she watched the teenager be stabbed at least three times and was so shocked she initially told police it was a shooting.
The court was told how Mr Headlam was chased before being caught and stabbed in the legs.
A pathologist told jurors he suffered two fatal stab wounds to the right thigh and a third wound to the back of the left thigh.
It's claimed the teen was heard screaming 'don't stab me, don't kill me' before being murdered.
Bishop Alonzo Headlam, is leader of the World Vision Apostolic Church of Jesus Christ, and was seriously ill at the time of the incident.
He suffered a stroke three months before the death of his son.
Idris Daud, 19, T'Shai Ennis, 19, and Ayman Koshin, 18, of Summers Close, all deny murder.
The trial continues.