just had a thought. If the cop thought he had a potential armed robbery suspect, I would think, standard procedure would be to call for and wait for back up before even getting out of his vehicle and approaching.
I think that's a good question, but the police scanner recording (if it's legitimate) indicates that he was only going to do an ID check based on the resemblance of Castile to the suspect.
Also, there were actually two officers in the patrol car. So, he wasn't alone.
Cop was a small Hispanic guy, Doubt he get much respect at the station. This is his big chance to be the hero.