Empire State Building shootings
NYPD officers confront the killer, and seconds later he is dead and nine bystanders shot and wounded. [corrected]
It would have been even worse if the perp had gotten a shot off.
Granted, marksmanship goes down the tube when you're looking down the other guy's barrel, but if a CCW holder had responded with this outcome, it would forever be cited as proof that civilians acting in self-defense are more danger than they are worth.