To those who are criticizing the OP, bear in mind the Clint Eastwood line, "A man's gotta know his limitations."
A CCW type who chooses to engage an active shooter can easily put others at risk.
1) He could misinterpret a situation, and engage a victim who was already engaged in lawful self-defense;
2) She could misinterpret the situation, and engage another armed Samaritan or a plainclothes cop;
3) He could engage a valid target, but miss and end up shooting innocents;
4) She could engage a valid target, lose the gunfight, and draw the shooter's attention and wrath to her own family.
So, as Pax and others have noted, the correct answer will vary. The nature of the scenario, and the perceptions and abilities of the CCW carrier, should affect each decision.