In military actions prior to the Crimean War, the ratio of dead to wounded was usually around 1:1. After the 1850s, medical corps were formed and more of the wounded survived. By WW2, 1:2 ratio was common, and modern US army is closer to 1:4. In other words, people hit by military weapons are still far more likely to survive than to die. The factors contributing to survival are usually protective gear, good physical condition of the troops, and prompt medical attention.
In modern times, we know that people shot with handguns have an 85% chance of survival, provided they get medical treatment. The chances are far lower with shotgun or rifle, but even then modern medicine can accomplish near miracles. Given the proximity of most mass shootings to urban medical centers, how do we end up with statistics reflecting dead:wounded ratios opposite of the military numbers? How is it possible that a single person with a hand-held weapon could produce more deaths than non-fatal injuries? The answer is simple: control of the scene.
A murderer who has the crime scene to himself can take the time to shoot or stab every victim again, just to make sure they are dead. That’s been the typical mode of operation for the killers: shoot a group of victims, then backtrack and shoot everyone in the head. The other factor is the exclusion of medical emergency response personnel from the scene, meaning that the victims bleed out before anyone reaches them. Those two factors, combined with fear-driven compliance and resulting close-range shots on non-evading victims (likely the reason why the school attack in Newtown had 27 deaths to one injury), are the reason why murderers with seemingly light weapons are so efficient. Given control of the scene and lack of resistance, a 1770s regular soldier with a musket, bayonet and sword would have killed as many, primarily with the blades.
The emphasis on shooting back comes not only from the likelihood of stopping mass murderers dead before they can harm more than a few victims, but also because even suppressive fire distracts them from the initial quest. In all recorded incidents, attackers who came under fire — even when the return fire was ineffective — stopped what they were doing and either hid or tried to fight the responders. As a result, the wounded had much better chance of survival.
Immediate armed response is also important for avoiding friendly fire. A person present at the start of an incident are much more likely to know who the aggressor is than would responding cops.