I will say that more often than not having an armed bystander intervene in an active shooter situation doesn't end well, I think it has the potential to be helpful. People that have a license to carry should also take courses on what to do if they encounter an active shooter situation. I think if more people did this we would be safer overall.
Responding officers could easily confuse the gun-carrying civilian with the armed assailant and may return fire. It would be best for civilians to focus more on reaching safety, warning others, and calling for help. Also, the average civilian is not trained for such combat and could easily harm bystanders with friendly fire.
When dealing with an armed gunman the last thing that the police need is an unknown person with a gun throwing themselves in the mix. The police don't know this person and they don't know if this person is going to put themselves or others in a dangerous situation. It is a dangerous variable that can get more people killed.
As this incident goes to show, it is never a good idea for a civilian to embroil themselves in an shooting incident whether or not they are themselves armed. Carrying a gun does not mean that you are protected against being injured or killed by a gun toting maniac and gives a false sense of confidence. Leave such interventions to professionals and stay out of the incident.