A teenager has told a jury he does not know why he stabbed a nurse to death.
Cheryl Moss was stabbed during a cigarette break
Stuart Harling described ambushing Cheryl Moss, 33, as she took a break near woods behind St George's Hospital, Hornchurch, east London, in April 2006.
"I remember stabbing her in the back while she was on the floor there," the trainee accountant told the jury. "It was like seeing a film."
Mr Harling, 19, from Rainham, admits the killing but denies murder on the grounds of diminished responsibility.
He spotted the nurse having a cigarette in some woods behind the hospital and waited until she finished speaking on her mobile phone.
Wearing a wig and carrying a Crocodile Dundee-style knife, Mr Harling then ran towards her.
Mr Harling said: "I thought I would try to get a glint of the sun on [the knife] so it was clearly seen.
"She noticed me and she said 'oh no'. I was still running. I started stabbing her in the back."
Mrs Moss, 33, who was stabbed 72 times, was already dead when she was found by her colleagues.
Mr Harling said he had been been trying to steal a car as part of a plot to overthrow an African government.
He said he had staked out the hospital car park and had planned to threaten the nurse to get her car keys.
The trial continues.