When you outlaw things, such as guns, you outlaw them for the honest law abiding citizens. Stripping those who would use these arms of their means of protection. This enables all people of a less than admirable disposition the upper hand in weaponry. It gives criminals the terrible knowledge that, thanks to their black market connections and disregard of legalities, they will not be "outgunned"; thus giving them clearance to heighten the level and frequency of their illegal actions with minimal resistance.
Making guns legal would lead to a great increase in gun related violence. Countries where guns are legal have a much higher rate of murder and shootings. There are no good reasons for guns to be legal again. The police and military are the only people who need to be armed in a modern world.