Your premise is wrong. Armed citizens & sworn LE officers get shot or killed all the time.
An armed off duty police officer(with over 20 years service) was killed in a ATM robbery.
No one, even a armed citizen has a absolute ability to survive a lethal force event.
Ah, but you failed to recognize his premise: It takes a government to put the "mass" in massacre.
Individuals kill at a retail level- one here, maybe a couple there. The most prolific indvidual mass murderers in history have never killed more than Vasili Blohkin did in a single night (and he did so for 28 straight nights
It takes a Government to kill on a wholesale level ...... first by disarming the victims, and them by systematicly killing them. Armed people with a will to resist can NOT be lined up in front of a ditch and machinegunned, marched into a gas chamber to be asphyxiated, or herded into a church to be burned alive.