The man accused of two murders near bus stops said a staff member carried out the attacks, the Old Bailey heard.
Amelie Delagrange was one of two women murdered
Levi Bellfield, 39, claimed he saw one of his employees, Sunil Gharu, pulling victim Irma Draghoshi, who survived, in the December 2003 attack.
Mr Bellfield, of West London, denies two counts of murder and two of attempted murder between 2001 and 2004.
Two women, Amelie Delagrange, 22 and Marsha McDonnell, 19, were killed when they were hit on the head.
Mr Bellfield - who ran a car clamping firm - said did not know what was happening to Ms Draghoshi but was later sickened when he found out the extent of her injuries, he told the jury.
Ms Draghoshi, 33, suffered head injuries when she was attacked while waiting for a bus in Longford village, near Heathrow airport.
The prosecution alleged Mr Bellfield knocked her to the ground as part of his string of attacks on unsuspecting women, two of which ended in murder.
Mr Bellfield is alleged to have driven after buses and to have attacked women waiting at isolated stops.
But Mr Bellfield, on his second day in the dock, said that on the day in question he came back to his car to find Mr Gharu had left the vehicle.
"He was pulling at the lady. As he pulled a handbag or her arm, she fell back and that is when I drove off," he said.
The defendant demonstrated to the court, using both of his hands and pulling towards his body.
'I feel sick'
"Part of me was in a bit of shock," he said. "I just did not want to get involved. I could not believe what happened."
Mr Bellfield told the court Mr Gharu said to him Ms Draghoshi may have been involved in robbing his girlfriend of her mobile phone.
He added: "I did not know Miss Draghoshi's injuries. Looking at this lady's injuries now, I feel sick."
The defendant said Mr Gharu and his brother were some of as many as 20 men who had access to vehicles used in his car-clamping business.
He said Mr Gharu and his brother were in a Toyota Previa at the time schoolgirl Kate Sheedy, 18, was run over.
Mr Gharu gave evidence in the trial and denied Mr Bellfield's allegations.
The case continues.