I imagine this senerio:
24 hour quicky mart gas station. I'm in back looking for something to quench my thirst (I love gatorade) because I've been on the road for a long time. Armed robber runs in, points a gun at the clerk, and demands cash. I'd probably cover, draw, and then yell "PUT THE GUN DOWN!". What happens next is up to the robber. If he turns the gun on me I'll drop him.
So he turns on you, raises the gun and you drop him, to find out he was wielding an airsoft pistol?
Even if the robber was armed, it could be tough to justify a homicide in the scenario described. It's way outside of castle doctrine. You might get away with it in Texas. RCW 9A.16.110 might help you in WA but it would be ugly.