The following is just my humble opinion:
1. Depending on the type of operation, terrorists may or may not be trained for combat. Bombers who have explosives strapped to them will likely not have effective training. But those who require training to ensure a completed mission will likely have solid training and very thorough planning.
Example is ofcourse the 9/11 hijackers who not only did practice runs on the very aircraft they would hijack, but also carried and used pepper spray to deter people from fighting back, and were trained in martial arts in case people did fight back.
2. As we have seen in the middle east, the favored weapon of the terrorist isn't an AK-47, it is an explosive device. Why shoot 10 people when you can blow up 45?
3. If a terrorist group does do a shooting at a mall in the USA, you can be sure that there are also explosives involved. Either set on timers for the rescue personel, or at the exits to catch the evacuating shoppers.
4. Going after multiple trained adversaries(see #1) who are armed with long arms while you are armed only with a pistol and maybe a reload or two is suicide.
5. If you do fight back and take one of the terrorists weapons, you will be easily mistaken as a terrorist as well. Responding officers or other CHL holders might very well take you out.
6. The advice to get down a narrow hallway is a good one. You have concealment (if there is a corner), and the element of suprise is back on your side if they come down the hallway. Also, often times these little hallways that lead to bathrooms also lead to small exits.
7. Avoid the main major exits. If the terrorists want to inflict more casualties, they could easily place explosives there.