Look, the guy was one-man SWAT op. He had people inside a dark box with one or two narrow exits, threw some gas in there, and then came in firing, while fully armored and wearing a gas mask. A pro wouldn’t be able to do anything in this situation other than maybe tackle him if he was sitting close enough and before the gas makes him as useless as anybody else. That’s how terrorists and hostage-tackers get taken out, and they are armed themselves. Would any one of us feel better about having a gun in that situation? Probably. Would it help? Most likely not unless you plan on also carrying a gas mask yourself, along with a bullet-proof vest.