For me it is very simple. Defense is first and foremost the responsibility of the individual, and a firearm is in some cases the best line of defense. Furthermore, even if it were possible to magically remove all firearms from the hands of criminals, it should be an option for citizens to be able to own and carry a weapon. As the old saying goes, "God created all men, but Samuel Colt made them all equal." A criminal does not have to have a gun in order to be a deadly threat, and too often would be able overpower and possibly kill an innocent person using another type of weapon or even their hands. A firearm allows people to have a chance in these situations, either giving them an advantage over the criminals or at least putting them on equal footing.
I cannot speak for other countries, but it is estimated that in the United States armed citizens stop an average of 2,000,000 crimes per year, often by simply drawing their weapon, as shots are never fired in many of the cases. These are 2,000,000 crimes that would have taken place if citizens did not have the option of arming themselves. The two most recent shooting in my area, for example, where defensive shootings that stopped crimes in progress. One took place when a knife-wielding teenager burglarized a home, realized the elderly owner was inside, and then tried to slit the old man's throat. However, the owner brandished a rifle and shot the intruder several times, giving him a chance to call the police and to have the intruder arrested. In another, a group of armed men broke into a house, tied up a family, and threatened to kill everyone. Some alert neighbors took notice of the comotion and went to intervene, with one shooting one of the intruders. Now the family is safe, and the shot intruder is in policy custody and cooperating with police to capture his accomplices. Cases where a simple draw down sufficed are reported by the media less often, however, as they are not as newsworthy.
Many areas in the States seek to restrict firearm ownership and usage as much as possible, whereas others are liberalizing their gun laws. Guess which ones are making it easier for criminals. It's not the former. Texas, for example, passed a law ten years ago that allows citizen to carry concealed handguns if they obtain the proper license. Skeptics warned of blood flowing through the streets and Wild West shootouts occuring on every corner. Not only has this not happened, but, together with our "Make My Day" legislation, Texas has become an even worse place for criminals. Gun crimes still happen on a regular basis in places like Dallas and Houston, but these are mostly gang-related in the inner city. Another example is Oklahoma, where the passage of legislation allowing concealed carry has actually been proven to have had an effect on curbing gun-related violence.
That is not to say that firearms are for all law-abiding citizens. Some feel safer without them, especially people with children, and that is their choice. Others do not feel the need to have one, and that is their choice as well. Certainly staying out of potentially dangerous situations in the first place goes a long way. Gun ownership carries a great deal of responsibility, which includes owners educating themselves about their weapons and practicing their skills. People that cannot be bothered to do this could put themselves in greater danger by owning a weapon. Like any other tool, a firearm can only be used safely and effectively with the proper education and training.
Nevertheless, it should be every individual's choice to make. Not everyone is willing to bet their lives on neither the possibility of being able to notify the police in time nor the compassion of criminals to spare their lives.