"...when the shooting starts there is no reason to shoot him."
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.