"He said in chilling terms that if someone was stupid enough to have a go and try to stop him then they got what they deserved," Martin Heslop QC, for the prosecution, said.
He alleged that unemployed Ian Kay, 28, had regarded his 22-year-old victim, John Penfold, as "fair game and a necessary hazard in a robbery". Mr Heslop added: "He said he had no regrets for what he had done."
Kay, of Bracknell, Berkshire, has denied murdering Mr Penfold on 4 November last year. He admitted a series of robberies,
Mr Penfold had worked for Woolworth's on Saturdays when he was 16, then joined the staff. He was appointed assistant manager at the Teddington branch, south-west London, the previous August, the court was told.
Kay was in debt when he set out to rob the store which he believed was an easy target, with low security and a young staff, Mr Heslop alleged.
Earlier the same day he had tried to rob a female cashier of another Woolworth's branch in Yiewsley, west London. He went to the till to pay for a packet of crisps and as the cashier turned he grabbed about pounds 200 in notes, the prosecution said.
But the cashier "with considerable courage" grabbed the notes from his hand and shouted for help. "Kay raised his hands in a gesture of surrender, left the money and ran out of the store. He had a small knife in his hand," Mr Heslop said.
He was annoyed at the failure of the first robbery and went off looking for another easy target, Mr Heslop said. He ruled out the Woolworth's at West Drayton as there were too many police around. "Eventually and unfortunately for John Penfold, he picked on Teddington."
Kay claimed he warned he would stab anyone trying to prevent him from stealing and showed his knife, Mr Heslop said. Mr Penfold collapsed shortly after he was stabbed. "It was a quite deliberate, cold and calculated stabbing intended to facilitate the robbery and his escape. In interviews with the police, he admitted he carried the knife for just that purpose."
The court was adjourned until Monday.