I think that's a fair question. The armed citizen *may* have saved dozens of lives (Hawk say hundreds), or maybe not. Can you make a citizen's arrest in Missouri for disturbing the peace? (probably not, but almost. I think DTP is usually a high-level misdemeanor) But if a reasonable person thought Rifle Guy was imminently going to shoot someone, he could use deadly force to stop him. (IANAL, especially not a Missouri criminal lawyer) Was Rifle Guy actually pointing the gun at anyone, or was it slung?
The fireman will be fine because no prosecutor will prosecute. But I think he may be on shaky ground and got lucky. At least he didn't shoot the guy.
What I read said the rifle was slung the whole time and he never showed any threatening behavior other than simply open carrying a rifle.
At another time, his behavior might have raised eyebrows or they would have asked him to leave the store. In this case, the manager assumed a copy cat shooting.