I stand corrected. There was at least one other cruiser on the scene when the officer pulls up. You can also see an officer flanking the victim while the other one is following him.
As for the officer who did the killing, he was just arriving on the scene.
If we're to believe what the DA is saying, then there are at least two officers on the scene that have a much better understanding of the situation. They have been there since before the guys went into the store. Which means they where also there when they came out. Unless the three guys standing by the truck weren't the ones in question. No where has it been stated that the victim was actually one of the guys suspect of having a weapon, and the officers that knew who the suspects were had not approached the guys standing by the truck. They were supposedly waiting for the suspects to come out of the store.
Why does this officer pull up to a controlled situation with officers already on the scene, fail to first discuss the situation with the officers who have a much better idea of what is going on?
Instead, he pulls up and immediately escalates the situation.
One important detail that is rarely mentioned: NONE OF THE GUYS HAD A WEAPON.