Mr Simpson was killed after the store closed for the day
A security guard was "instrumental" in planning a failed robbery at an east London Matalan store during which the manager was killed, a court heard.
Jamie Simpson, 33, was stabbed in Kingsland shopping centre in Hackney in March 2008, as he was cashing up the day's takings, the Old Bailey heard.
Simeon Jumah, Kobina Essel, Anthony Maina, and a 17-year-old boy admit conspiracy to rob but deny murder.
Guard Roy Williams, Jamal Chambers and Duane Owusu deny both charges.
"Ringleader" Jumah, 25, of Manor Park, east London, had planned the raid with Mr Williams, 31, of Upper Edmonton, north London. Jumah was expecting to get up to £30,000 in takings, the court heard.
"Williams's role in one sense is even more sinister. He was the security guard at Matalan - in other words, the inside man," Aftab Jafferjee QC, prosecuting, said.
On 22 March, Essel, 18, Limehouse, east London, entered the store with Maina, 20, of Canning Town, east London, and the 17-year-old from Poplar, also east London, who cannot be named because of his age.
When confronted by the 19-stone Mr Simpson, of Herne Hill, south London, it was Essel who stabbed him with a "fearsome weapon", Mr Jafferjee said.
"Jamie Simpson, when confronted by Essel, did not yield, and he was stabbed to death by Essel. The robbers then fled empty-handed.
"Jamie Simpson had been entrusted by the store to look after the takings and he did so with his life.
"With the assistance of security guard Williams and his inside information, each of these defendants would have known by then that what would stand between them and the store's money was the duty manager Jamie Simpson."
Mr Simpson's family, from Derby, attended court for the trial.
The trial continues.