Keep in mind that only about 3% of age eligible people nationally have a permit. And some portion of those, probably larger than we'd like to think, don't actually carry with any regularity. So a perp's odds of encountering a lawfully armed victim on the steet are say, 1 in a hundred, at worst. And that will vary wildly by location. In Chitcago or NYC or LA or DC - odds are virtually nil.
Yes, you see reports of crimes every day. Some in recent times seem to have stepped up the violence level. Yet if we are to believe the compiled stats the FBI puts out, overall violent crime has been steadily decreasing the past couple decades. Whether the increase in permit holders has made an impact, we can't really know that.
All we can be sure of, and we need to keep this fact front and center, is that with the increase in people lawfully carrying handguns, there has not been a flow of blood in the streets, there have not been duels in Main Street. Those with permits to carry are by and large much less likely to commit a violent crime, or any other serious crime. But then, that comes from permit holders being a self selected group who have shown a history of being law abiding people.