...For starters, the article says the two observed the victim being held at gunpoint. There is already enough justification for deadly force in ANY state of the union. But you said they chased the dude, then exchanged gunfire with the dude. You also said the robber did not physically harm the victim. Guess what? The law does not mandate someone 1st inflict physical harm before you can use deadly force - that gun pointed at the victim was enough justification to immediately shoot the robber if the situation called for it.
We don't know if the robber pointed the gun at the other two, but it does not matter.
I am not sure what (else) you would like to discuss...
and hey, I am out of this thread.