Under those circumstances it is reasonable for the officer to fear for his life. The suspect can turn and fire faster than the officer can react. The officer has a duty to pursue the suspect and take him into custody. The bystanders came up to the location and shouted at the officer not to shoot anymore. They also said the suspect did not have a gun.
The bystanders should have stayed out of the way and let the officer do his job. The bystanders are the reason the suspect was not given immediate medical treatment The officer can't know who's side they are on, they may attack him. The suspect may still be alive, and may still be a threat to the officer. The officer was alone at the time.