Guns are an unnecessary social liability.
1. The majority of homicides in the USA are deaths by firearm. 
2.) Although we cannot say for certain that the previously mentioned homicides would not have occurred were there no firearms, it is highly probable that many of these homicide victims would not have died, as it is far easier to kill someone with a firearm than a knife, a bat, etc.
3.) Household firearms significantly increase the probability of an accidental fatality , .
4.) It is a common right-wing argument that areas with strong gun control tend to have greater crime and this is because of the lack of guns in the environment. These arguments are faulty at best. How would the crime rate be affected if gun bans were lifted? Right-wingers say they would decrease because everyone would have guns. But the police have guns too, and they are very prepared to use them, and that doesn't stop criminals from committing crimes. More guns in the environment only increase the chances that they will get in to the hands of criminals. Burglaries, thefts and robberies are all means by which criminals could obtain firearms. Areas with strong gun control tend to have copious amounts of crime due to cultural and economic issues, not a lack of guns.
5.) You cannot justify keeping guns for protection. The police keep your community safe through deterrence (police presence discourages crime) and through quick responses to emergency calls. Pepper spray is just as reliable and far less dangerous than a twelve gauge shotgun. Are you telling that every time you hear a bump in the night, you grab that shotgun?
6.) It is often said that criminals will always get guns even if they are outlawed, but what if we completely banned the manufacturing of firearms, as we have banned the manufacturing of nukes? From that point on, firearms can be disposed of until they are so rare and expensive that it isn't worth a criminal's time and effort to try and find one. Also, most criminals cannot and will not manufacture reliable homemade firearms. The number of criminals out there who know how to make a working semi-auto firearm has got to be negligible. The chances that those who do will be able to produce homemade firearms at an industrial rate are slim to none.
You cannot justify owning firearms. We live in a different society from the one which the authors of the US constitution lived in. No sane, rational, intelligent individual would argue against gun control. Guns have no place in the hands of civilians and most police officers in civilized societies.
"The NCVS remains the most authoritative source on the criminal and defensive uses of guns, and it shows that these weapons are overwhelmingly used more for crime than self-defense."
 Department of Justice, Federal Bureau of Investigation, Uniform Crime Reports for the United States, 1997 and 2007.
 Wiebe DJ. Firearms in US homes as a risk factor for unintentional gunshot fatality. Accident Analysis & Prevention 2003;35:711-6.
Wiebe DJ. Homicide and suicide risks associated with firearms in the home: a national case-control study. Annals of Emergency Medicine 2003;41:771-82.