Training and having a plan is important but a situation like a terrest threat is more than most of us can deal with. Picking up a bad guys weapon may be a very bad idea because the LEO's will assume You would be the threat.
If I would get stuck in a situation like that ,I sure would want an extra mag or two. Just to fight my way out.
Events like this shouldn't happen until the next administration takes over and will happen in one of the "Sheepherder" cities, softer targets with less CCW presence. Terrorist plan ahead and want their event to be dramatic as possible. Televized sporting events, conventions, press conferences and other high profile events. Testing the leadership early into their term. That's something to think about !?!