If he does not pose a deadly threat you better not shoot him.
You can take solace that when they catch him they will charge him with first degree murder for the death of his thug friend.
Personally, I would rather he go to jail for that than I go to jail for an unjustified shoot.
You could say that you thought he was armed because his partner was armed. You might get away with it but it is better not to take the chance if he is unarmed.
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.