Does Carrying a Firearm Keep You Safe?

Many of us believe that by just owning and carrying a gun keeps us safe. So now that you own and carry a firearm, are you really prepared to protect yourself and possibly others? 
This question reminds of an incident in law enforcement training that effected my way of prepared thinking for the rest of my life. I remember sitting in class with other cadets while learning Illinois Traffic Laws, and then out of nowhere someone ran into our class room with a hand gun, made some aggressive remarks to our instructor, and then proceeded to take 3 close range shots at our instructor. The perpetrator then fled the room.
During the event, while some of the cadets took comfort in hiding under their desk, others such as myself just sat at our desk with the look of deer in headlights. It was then after about 5 seconds, when I witnessed a couple cadets run toward the classroom door to pursue the perpetrator, while myself and a couple of other cadets went to check on our instructor who was lying on the floor. It was then right before the pursuing cadets reached the door, when our instructed from appeared shooting death yelled, "FREEZE!!!". He then picked himself off the floor, and proceeded to tell each of us to pull out a piece of paper and write everything we witnessed. He wanted a full description of the perpetrator, and everything that occurred (what, when, where, how).
To no surprise we all failed. Failed not in only in our observations, but later video evidence showing us how we failed miserably in our reactions. To say the least, we were nowhere near prepared.
It was this test and test that followed that completely changed me. I could longer go into a Walmart, McDonalds, or Gas Station without tracking everyone present. As soon as I would walk into a building, I would immediately look for accessible entrances and emergency exits. I could no longer sit with my back to an entrance, and I became astute at noticing odd unusual behavior, and areas containing unsafe environments.
Though I changed, I would look back at the class room event, and I would continue to ask myself "Am I prepared NOW?". It wasn't until later when I spoke to a firearm instructor friend about the question that I finally knew the answer. He told me, "Your definitely MORE prepared than most others, but there is NO way to be prepared for every situation at all times.".