Clearly you have never investigated a robbery before, the way those things work is the gun is used as a control method, they point the gun at the victim and demanded all their money and valuables. Usually the victims is so scared of possibly being shot they cooperate.
In this case the person just shot him in the back, this is extremely rare. He never gave the target an opportunity to see the gun and comply with orders to give up his valuables, but even after shooting him, he did not actually take any valuables, supposedly the reason for the operation in the first place?
The shooting seems to be the actual goal of the encounter, not a robbery.