Today things are a little different. If you assault somebody most likely you will not be caught. If you are caught, because you carried out your assault right in front of a police officer, you will have nothing more than a wrist slap. A far greater risk than in the past. It is also the commonly held view that if you defend yourself it will be you, who has just been assaulted, that will end up in serious trouble while your assailant walks away. So many victims are less likely to try and defend themselves, again reducing the risk for the assailant. As the risks decrease it will become less than the potential rewards of unprovoked violence for an increasing number of people, so you will get more.
With the state unwilling to fulfill its duty to protect its citizens the natural, and correct, response of the citizens is to try and defend themselves. This is why there has been an increase in the number of knifes taken into schools. They are not there as tools, as in the past, but as protection. Trying to use the blunt instrument of the law against people carrying knifes will make no difference unless they tackle the root of the problem. Which is the growing feeling that if somebody actually uses a knife against you, rather than just carry it, or a bottle, or their boots, or their fists then the state will look the other way and try to ignore it because it is easier and safer to go after the generally peaceful such as motorists.
It is not only the government that has failed here, it is the police too; the response of the police to these problems is simply to ask for yet more powers to detain, search, stop, arrest and the rest. They have deliberately turned away from the detection of serious crimes in favour of hitting easy state-set targets.
The first thing that any government should now do is to simplify the law along libertarian lines and to allow the police to do the job that we want them to do, i.e. to protect life, liberty and property and, when unable to do so, to find the perpetrators. We then need the courts to impose incredibly harsh penalties.
The decision to commit a crime requires a criminal to assess the situation via a cost/benefit analysis; if the chances of being caught and heavily punished are high, then they will add to the cost side substantially—enough, hopefully, to outweight the benefit.