Not by police, but a bystander

Shot by an armed bystander in Boston. A psych patient attacks a doctor, and is shot by an armed, off duty security guard. Not an on duty or off duty cop, but a guy who was armed. Carrying a pistol.

Being a sheepdog, rather than a sheep

Possibly saving the doctors life (stabbed several times, but alive and recovering at this time) by shooting and killing the psycho psych patient. Even though most people in Massechussets are not allowed to be armed (Restrictive May Issue), this man was armed due to his profession, and he was in the right place at the right time.

How many more killings, robberies, or muggings could be stopped if more citizens were armed and able to stop criminal acts?