I think the crucial thing is to not fall into thinking of a firearm as a talisman that can ward off evil. You do have to train and practice with them. Handgun shooting in particular is a perishable skill, you use it or lose it.

The one thing that I can say for sure is that hiding one's head in the sand and pretending things will go away is ineffective. It's magical thinking. The odds of being in a shooter are relatively low compared to some other threats we face ever day. But an active shooter situation is a "Black Swan", an event with a low probability but potentially catastrophic results.

We all make decisions about risk vs the efforts and costs associated with mitigating them. Look at something very routine like buckling your seatbelt. The risk of an accident for any given single trip is pretty low. The potential harms of being in an accident and not being buckled in though are serious, potentially fatal even at just above fender-bender speeds. Since the effort required to buckle up is minimal every responsible adult wears their belt. Other risk analysis situations are more complex. For example, do you buy insurance for your cell phone? How many fire extinguishers do you need for your home? Do you pay to do a criminal records search on your babysitter or nanny?

Carrying a gun is one of the most complex decisions to analyze. A good many people will go their whole lives without being faced with violent crime. Carrying a gun has costs; the gun costs money, you must buy ammo, you need to train (some places require it for getting a permit) and you need a holster. There's a cost in wardrobe to dress for the gun. Of course, a gun is dangerous- that's why I own them. But used carelessly they're dangerous to the owner and her family.

Not carrying a gun also has its risks. It's difficult to really know the actual risk you face ever day. The staff at Charlie Hebdo knew there had been threats and even attacks, but realistically how could they know that they'd be attacked by terrorists with AKs, dynamite and RPGs? You just don't know.

Crime can be one of those events with perhaps low daily individual odds but very serious consequences. You might lose property, you might be killed. You might comply fully and still be killed. It's dangerous to rely on the good will of someone that's demonstrated the willingness to murder you for the $20 you have in your wallet.