I just saw a video of The Daily Show's Jordan Klepper "testing" the theory that good guys with guns stop bad guys with guns, with the inevitable scripted comedic failure. Apparently quite a few people believe that this is evidence debunking the concept, that this was a real test rather than a scripted comedy show.
I watch almost no TV, so I'm unfamiliar with Jordan Klepper--he appears to be playing the part of a stereotypical conservative blowhard. Even assuming this wasn't scripted and he was trying to prove rather than disprove, the scenario was among the most difficult for a marginally trained person to deal with--Few people other than SWAT team members are trained in clearing a building, and the scenarios used as SWAT training are deliberately more difficult than most real life situations.
Like most trainers, the one in the show thinks most people should have more training, and it was at least edited to appear that he claims that a lifetime of training is required to be effective. In every case I'm aware of spree shooters have no more victims once someone else shoots towards them--even if the spree shooter isn't hit. I have yet to hear of a good guy with a gun making things worse (for anyone but himself in one case) in a spree shooting. It isn't plausible that the various gun control groups would ignore such a case if one existed. (I also think it more likely that the trainer's actual views are that a basic CCW class does not qualify you for SWAT duty, not that basic CCW is useless)
The trainer also said that very few spree shootings are stopped by armed civilians--I think he said around 3%. He didn't mention on camera that successful spree shootings rarely take place where it is legal for civilians to be armed--I don't know exact figures and they would depend on definitions, but I would be surprised if over 3% of spree shootings are where CCW is generally available and legal. The Gabrielle Giffords assassination attempt is the only incident I'm aware of where concealed carry was legal and widely available. California is one of the few remaining states where carry licenses are issued at the discression of law enforcement rather than based on objective criteria.
Few if any gun advocates claim that a gun will solve all violence, or that everyone should be armed--most of us think that if you don't want to be armed, you shouldn't be armed. Rather, the majority of people who have taken the time to obtain a license have reasonable expectations, and on balance will do significantly more good than harm even if they don't succeed every time.