"The only thing that stops a bad guy with a gun is a good guy with a gun"
1) "Only" and "stops" simplifies an argument that isn't about eliminating gun violence, but reducing it.
2) Sometimes good guys kill other good guys, by mistake, and that's a problem too
3) Sometimes an otherwise good guy becomes a bad guy when intoxicated or in a bout of passion (see http://nyti.ms/VHE1Jo )
4) What about reducing the access of bad guys to guns in the first place, even if it inconveniences the good guys? (longer background check, universal background check, mental health records at state level required for all background reviews, limit on number of gun purchases per person per unit time, etc)
5) You notice that you can't really say "The only thing that stops a bad guy with a knife is a good guy with a knife". Meaning that just by applying this good-guy bad-guy construct to guns, the NRA is implicitly recognizing that overall homicide rates are higher because of guns being easy to use, ranged, and lethal. Oops!