AK
Yes it happens. The use of guns by "armed citizens" against criminals is not a big percentage of the "war on crime" these days, but it does happen. Not a big news story though.
There are also a lot of "normal" people that take things into their own hands and beat, bash, poke, claw or otherwise repel violent criminals without the use of guns.
However, as the likelihood of the above drop to "just not going to happen" in the mind of criminals, the only deterrent to violent crime becomes the police / military.
We know who is in charge of the police /military. Given that the government never makes bad choices about protecting the safety of the law abiding populace, wide spread violent crime is sure to just disappear any time now (sorry, but I don't feel like holding my breath on the above postulation).
In some area of the world where "big govenment" is non-functional or otherwise misguided, local areas still remain relatively calm and well mannered. The "moral fabric" of these local areas is probably more important than anything else. I have to believe that where you have a good "moral fabric" you have a higher fraction of people that will stand up to bad guys and you have more options for youth to avoid the path to a life of crime.