From the Sacramento Bee:
Mike Harris has similar views. The former Marine Corps drill instructor said he carries a concealed pistol, a habit he picked up after being robbed when he was the owner of several Sizzler steakhouses and frequently carried cash from the day’s receipts.
But he considers himself a trained and knowledgeable gun owner.
Harris was at the scene of the shooting because he wanted to visit a friend who works for Giffords. He arrived at the Safeway parking lot just moments after the gunman had been restrained.
Harris immediately began performing CPR on victims. He never thought of drawing his weapon, he said, because others had detained the shooter.
“Afterward, my wife said, ‘What would you have done if you’d gotten there earlier?’ I would have killed him,” Harris said.
This article appears in Jan 13-19, 2011.

great , the gun nuts are going to save everybody with their guns – NOT!
so, what is the first sign he would notice of something going wrong if he was there earlier?
if he was lucky, and he saw the perp rushing thru the crowd, he might of have mitigated some of the slaughter.
But I think it is more likely that his first sign would have been the gunshots.
And how long did it take to fire all the shots?
30 seconds?
he would have to notice something wrong, get over the momentary shock of it, get in range, draw his weapon, and then…?
My point of all this is that, by this time, most of the carnage would have already occurred before he fired his first shot.
Trained security scanning the crowd would be a much better idea.
i agree with anon [above] and would add;
what would insure Harris is a good enough marksman to hit his target , under stress, in a crowd! what prevents other persons, also carrying firearms at the scene, from identifying Harris as another perp and opening fire on him, in a crowd!
handguns more often miss than hit their targets and qualified shooters know this and unless the shot is perfectly placed will not stop a determined attacker from continuing.
with a group of gun carrying amateurs , using their weapons to try to subdue a shooter in a crowded area the carnage of innocent bystanders could be manifest.
i am not in favor of gun control, but do favor the reduction of the unrealistic bravado perpetuated by those who claim their possession of a firearm makes us all safer.
The thing I keep coming back to is that if the shooter had not been captured alive and instead was shot dead at the scene then it would have taken a longer time to figure out if he was acting alone and/or if his attack was isolated to what had already occurred.