Police say two men entered the store just after 7:00 p.m and began arguing with a store security guard -- identified by police as Bounhom Manyvong, 59, of Oakland.
Moments later, the men pulled out a gun and shot Manyvong three times inside the store. Before fleeing from the scene, the men stole Manyvong's gun.
Paramedics transported the security guard to a local hospital by ambulance, but he eventually succumbed to his injuries.
Manyvong's family and friends told NBC Bay Area that Manyvong wasn't supposed to be at work Saturday -- he was filling in for someone who had called in sick.
The security guard was no stranger to the dangers of his profession; Manyvong's friends say he was robbed while working at a local Bank of America branch about four or five years ago.
An Auto Zone employee told NBC Bay Area that the store is notorious for shoplifting, though it appears the suspects in this incident didn't steal any merchandise.