Active Shooter Scenerio: Can Armed Citizens Save Lives?

Every time there's a shooting with an active gunman, the discussion  comes up. Usually online.
Would an armed citizen be better or worse if added to this type of situation?

"There were kids in my department, families mom's. we locked ourselves in the dressing rooms."
When a gunman opens fire at a Portland area mall there is chaos and terror.
"i just want to hug her and ...."
yeah, active shooters are very very dangerous. the most  for serious, the most serious that we'll ever encounter in law enforcement."
After the shooting, many posted on social media sites including ours suggesting an armed citizen could have prevented loss of life.
"it would be rather risky to i think try an engage somebody like that."Josh Evans says on out Facebook page:
"This is why more everyday citizens need to have firearms training. Maybe instead of running like cowards someone could have fought back and killed him before he shot too many people."
Another user replied.
"Gun advocates say a person carrying a concealed weapon may be able to assist if no one else is around to help."  
Amber Hatfield
"I fully support carrying concealed and defending yourself in times like this." But, police say the situation is very dangerous, adding another gunman could only make it worse.  Bottom line- it's safer for you and those around you to escape rather than try to shoot