So you’re recommending that we assume the second robber was unarmed?
If two masked men walk in together to rob a business and one of them brandishes a firearm, a reasonable person will not assume that his partner is unarmed... And that’s the standard. Would a reasonable person have believed that their life was in danger? Yes.
Would I rather be judged by twelve than carried by six? Yes.
If the guy bolts as soon as the shooting starts you do not have to assume anything because he is no longer a theat.
If he is no longer a threat you can’t shoot him.
I assumed he was unarmed because he ran away and was not shot. But it really makes no difference.
If you want to go to jail, shoot a man in the back while he is running away.