To speak in NRA terms, if you kill a man who would otherwise kills thousands of people, it means you may have killed one but thousands of lives are saved, so 1 vs thousands.... Well, the math fits, but math does not always show reality.
In the end a gun is only as good and bad as the person who wields it, and that phrase does not only apply in ethic sense, but also, in the quality of the shooter. Civilian, only used a gun before on a dummy, no experience in life threatening situations, scared to death, everything to lose VERSUS fully trained mass murderer, you have no idea about his experience, has nothing to lose, ready to die (quite often even planning to commit suicide after the shooting anyway).... Sorry, as a realist my money is on the latter.
I once heard somebody ask a professional pirate hunter, if it isn't scary going into a ship hijacked by pirates all armed with fully loaded kalashnikovs. The hunter replied: "Only a fool wouldn't feel any form of fear, but you know that thanks to your training you are better than they are, regardless of the weapons they have." Sounds a bit arrogant, but when you realize I just quoted a professional in this field, you can see why these "good guys with a gun" can win against the "bad guys with a gun"... It's not their gun... It's their professional training.
And it also wouldn't be the first time that an unarmed man manages to "overpower" a man who was all packed up with guns, knives and explosives... Yes, sometimes even with violence... No matter how well armed you are, if somebody manages to sneak upon you from behind and manages to punch you knock-out on the moment you least expected it, then what will all your guns do?
And cocking a gun at somebody is easy, but a golden rule in gunfights is that once you cock your gun at somebody you give them the right to shoot you. And how many people have it in 'em to pull the trigger? One moment of hesitation and a real killer will pull theirs first... And then your gun is rendered useless in less than a second...
The math still fits, but math alone is not what it takes to fully discuss the need of guns or the need to outlaw them, is it?