See Slate’s complete coverage of the Gabrielle Giffords shooting and arrest of Jared Lee Loughner.

Does the Tucson, Ariz., massacre justify tighter gun control? Don’t be silly. Second Amendment advocates never look at mass shootings that way. For every nut job wreaking mayhem with a semiautomatic weapon, there’s a citizen with a firearm who could have stopped him. Look at the 1991 slaughter in Killeen, Texas, where 23 people died in a restaurant while a patron’s handgun, thanks to a dumb law, was left outside in her car. Look at the 2007 Virginia Tech massacre, where 32 people died because under the university’s naïve policy, nobody in the invaded classrooms was allowed to carry a firearm. Guns save lives. So the argument goes.

Now comes the tragedy in Tucson. And what do gun advocates propose? More guns. Arizona already lets people carry concealed weapons without requiring permits. The legislature is considering two bills to expand this right, and as Slate’s David Weigel reports, the Arizona Citizens Defense League is preparing legislation that would require the state to offer firearms training to politicians and their staff. The bill is tentatively titled the Giffords-Zimmerman Act in honor of the wounded congresswoman and her slain aide. “When everyone is carrying a firearm, nobody is going to be a victim,” argues the state’s top pro-gun legislator. Beyond Arizona, at least two members of Congress say they’ll bring guns while traveling their districts.

The new poster boy for this agenda is Joe Zamudio, a hero in the Tucson incident. Zamudio was in a nearby drug store when the shooting began, and he was armed. He ran to the scene and helped subdue the killer. Television interviewers are celebrating his courage, and pro-gun blogs are touting his equipment. “Bystander Says Carrying Gun Prompted Him to Help,” says the headline in the Wall Street Journal.

But before we embrace Zamudio’s brave intervention as proof of the value of being armed, let’s hear the whole story. “I came out of that store, I clicked the safety off, and I was ready,” he explained on Fox and Friends. “I had my hand on my gun. I had it in my jacket pocket here. And I came around the corner like this.” Zamudio demonstrated how his shooting hand was wrapped around the weapon, poised to draw and fire. As he rounded the corner, he saw a man holding a gun. “And that’s who I at first thought was the shooter,” Zamudio recalled. “I told him to ‘Drop it, drop it!’ “

But the man with the gun wasn’t the shooter. He had wrested the gun away from the shooter. “Had you shot that guy, it would have been a big, fat mess,” the interviewer pointed out.

Zamudio agreed:

I was very lucky. Honestly, it was a matter of seconds. Two, maybe three seconds between when I came through the doorway and when I was laying on top of [the real shooter], holding him down. So, I mean, in that short amount of time I made a lot of really big decisions really fast. … I was really lucky.

When Zamudio was asked what kind of weapons training he’d had, he answered: “My father raised me around guns … so I’m really comfortable with them. But I’ve never been in the military or had any professional training. I just reacted.”

The Arizona Daily Star, based on its interview with Zamudio, adds two details to the story. First, upon seeing the man with the gun, Zamudio “grabbed his arm and shoved him into a wall” before realizing he wasn’t the shooter. And second, one reason why Zamudio didn’t pull out his own weapon was that “he didn’t want to be confused as a second gunman.”

This is a much more dangerous picture than has generally been reported. Zamudio had released his safety and was poised to fire when he saw what he thought was the killer still holding his weapon. Zamudio had a split second to decide whether to shoot. He was sufficiently convinced of the killer’s identity to shove the man into a wall. But Zamudio didn’t use his gun. That’s how close he came to killing an innocent man. He was, as he acknowledges, “very lucky.”

That’s what happens when you run with a firearm to a scene of bloody havoc. In the chaos and pressure of the moment, you can shoot the wrong person. Or, by drawing your weapon, you can become the wrong person—a hero mistaken for a second gunman by another would-be hero with a gun. Bang, you’re dead. Or worse, bang bang bang bang bang: a firefight among several armed, confused, and innocent people in a crowd. It happens even among trained soldiers. Among civilians, the risk is that much greater.

We’re enormously lucky that Zamudio, without formal training, made the right split-second decisions. We can’t count on that the next time some nut job starts shooting. I hope Arizona does train lawmakers and their aides in the proper use of firearms. I hope they remember this training if they bring guns to constituent meetings. But mostly, I hope they don’t bring them.

Like Slate on Facebook. Follow us on Twitter. Human Nature’s latest short takes on the news, via Twitter: