If an armed civilian (and I'm talking genuine civilian, not off-duty cop or out-of-uniform military) tries to take out a well armed shooter, there are four possible outcomes:

1) He's successful.

2) The shooter specifically targets him because he has a gun and, thus, poses a greater threat to the shooter than anyone else in the room.

3) When police arrive, they see he's armed and shoot him, thinking he's one of the bad guys.

4) He misses the shooter and harms or kills an innocent person.

If I were a betting man and had a choice of putting $1,000 on Mr. Armed Civilian being a hero against a mass murderer and the Chicago Cubs winning the World Series, I'd go with the Cubs.