Self Defense vs. Defense of Others
The typical firearms instructor in most concealed carry classes teaches prospective armed citizens to distance themselves from the threat. This is the true concept of self defense; use the firearm to discourage the wolf from following you while you make your escape. In Grossmans analogy this would make them a dog, since they have the opportunity to fight back, but does not make them the sheepdog concerned for the safety of the sheep.
This brings up a legal issue that varies from one state to the next. In many places you must exhaust all available means of escape before the use of deadly force is acceptable under the law. Under this premise, potential victims in a mass shooting incident are all supposed to individually flee from the scene and wait for the police to eliminate the threat. They cannot fight back with deadly force unless retreat is not possible. In other locations the armed citizen need only feel threatened before using deadly force. In this scenario they do not have to try to run away first but may stand and fight when threatened. Using the mass shooting scenario, the armed citizen may use deadly force if the shooter enters their immediate vicinity.
This much information allows me to submit my entry to Grim's Virginia Tech war gaming of scenario 2. When the shooter enters the room I am in or begins shooting, I draw my reliable Kimber 1911, move to an area that reduces the threat to others in the room and allows for a clear line of fire to my target and repeatedly pull the trigger until the threat no longer exists then reload a fresh 8 round magazine. Just before the police arrive, I unload and clear my pistol, place it on the floor, and expect to spend the rest of the day at the police station.
Grim's scenario 1 is an entirely different situation. I ask; whose responsibility is it to protect others that face an immediate threat? Before you echo what would appear to be the obvious, the Supreme Court recently ruled that, "To Protect and Serve" is only a motto and not a binding commitment to the people. They stated that it was the responsibility of the police to investigate crimes after they occurred and they could not be held liable for not preventing a crime. In the VT scenario, the police were responsible for identifying the shooter only after he murdered his first victim. This obviously was not in time to prevent another 31 dead.
I am not aware of any location within the United States that has laws in place that would justify what I would do. Please be aware that my actions would be based on extensive training and superior marksmanship and even I would not recommend this course of action for the untrained individual. I would, in stark contrast to legal justification and the advice of almost all firearms instructors, advance toward the sound of the gunfire. I would find the classroom and peek in to identify the threat. With the VT shooter, I would have the superior firepower. I could have entered with confidence since single hits from a 9mm are rarely fatal and multiple hits are often required to stop an opponent but a single well placed hit with a .45 cal Federal HydraShock would make him ineligible as an organ donor. So, when his attention is way from the door, I would enter and move to a position that reduces the chances of others getting hurt and finish in the same manner as scenario 2 but would have to add my reliance on popular opinion, that my defense of others was justifiable, in place of legal justification.