The likelihood of a CCP holder just happening to be at the right place and right time during these massacres isn't all that great.
In the Colorado shooting incident the movie theater prohibited guns, even legally carried ones.
I don't necessarily subscribe to the "If only someone there had a gun" idea. No way to know when someone could have or would have stopped the gunman.
There are numerous occasions where armed civilians have stopped armed gunmen though, a recent one happened in a Florida internet cafe.