Average deaths in a shooting rampage when stopped by police: 14.3
Average deaths in a shooting rampage when stopped by civilians: 2.3.
It should be noted, though, that that includes unarmed civilians.
So really, a civilian actually doing something reduces the average from 14.3 to 2.6 — 11.7 people saved on average. Them being armed only saves 0.8 lives a shooting (1 life every 4 out of 5 shootings).
The point being: Doing something is better than doing nothing, and waiting for the police is what will give you the lowest odds of surviving. And if someone is going to be a hero – they will a better job at doing so if they have the tools available.