Studies from WWII that found that a a significant number (some as high as 75%) of soldiers would never fire their weapons directly on a person and the US military had to revamp a bunch of things to get the military to a point where like 90% of soldiers would fire at someone by Vietnam. Many self-defense gun owners are living a fantasy that they will be the Billy Badass that saves the day, but many, or even most, will freeze up or even run when a deadly confrontation happens, just like soldiers from WWII. It takes a certain inherent willingness to kill that most people lack (luckily) or special training to overcome the psychological barriers to killing another person, like the US military receives today, that most people will never have access too. Many cops are included in this group, if they are not veterans, or swat.
Adding more cops/security will not help much, as most will probably never receive the kind consistent training needed to ensure they can successfully square up on a shooter and take them out.