A gang of 12 men swarmed the store after the victim, 18-year-old Atiq Rehman, took shelter inside.
Rehman was chased around the store before being cornered and subjected to a savage beating. He died of serious head injuries two days later.
"The incident has been absolutely horrific," said store manager Thiru Kamalan.
"The whole community has been deeply distressed, particularly the staff who witnessed the whole thing. Many are now too scared to return to work," he said.
The store was forced to close for two days after the murder, and has lost almost £6,000 in sales.
"Closing was bad enough, but trade has been slow since we re-opened as many locals are worried about going out. We are at least 15% down," Thiru added.
The store also sustained major damage in the attack.
Three men have been charged with the murder.
Meanwhile, a Costcutter in Preston, Lancashire, was damaged in a suspected arson attack last week. Police believe a flammable liquid may have been poured in the letterbox.