The stories are out there, but they're hyped each way by members of each side of the argument. When a good guy does stop a bad guy, then the "arm everyone" crowd blows his achievement out of proportion, extolling how many potential victims were saved, while the "ban all guns" crowd totally underplays it - but then these two extremes tend to play in their own sandpits, that you have to hunt for.

National news tends to play down both sets of hype, so it tends to fall to local news "a local good samaritan prevented a bad guy from harming xxxx", it's easy to save that the samaritan probably saved the target, but difficult to quantify if the bad guy was doing general mayhem or just after the current target.

Arizona mall shooting - Dec 2012
So was the good guy a factor in the stoppage of the shooting? Did the bad guy just decide to end it because his primary weapon wasn't working? The end result was very few people were shot, so I don't believe it was that big of headline thing for the nationals.

Or there's this one from a couple days ago. Now it's not a mass murderer, but the guy definitely made a difference for the victim in this case. But once again it's just a local story, that's pushed on the gun supporter forums.
I used to think it was terrible that life was unfair. Then I thought what if life were fair and all of the terrible things that happen came because we really deserved them? Now I take comfort in the general unfairness and hostility of the universe.