The armed robbery occurred around 6:45 p.m. at a business in the 1500 block of North Sanborn Road.
The two suspects demanded money at gunpoint from the business owner and several patrons who were also inside, according to police.
The Good Samaritan, who was outside the business but was associated with one of the victims, witnessed the robbery and got a gun out of his vehicle to try to stop the robbery, police said.
Fearing for his safety when he entered the business, the Good Samaritan shot both suspects, who fled and were later taken to a hospital for treatment, according to police.
One of the suspects, a 17-year-old boy, was pronounced dead shortly after arriving at the hospital. His name is not yet being released.
The second suspect, also a 17-year-old boy, was listed in very critical condition this morning, police said.
The Good Samaritan fled briefly out of fear but eventually returned to speak with police. Following extensive interviews with him and the victims, as well as review of video surveillance footage at the business, police do not plan to seek charges in connection with the shooting.
Anyone with information about the case is encouraged to call the Police Department's investigations division at (831) 758-7226 or Detective Fors at (831) 758-7124.
People wishing to remain anonymous can call a tip line at (831) 775-4222, the We-Tip line at (800) 78-CRIME or can send a text message to 847411 with "SPD831" in the message.