A weapon can do nothing on it's own. A gun will just lay there waiting for someone to pick it up. The same goes for a hammer, a shovel, a pitchfork, etc.
The problem with banning weapons is that it's impossible. For countries like Japan that have bans on firearms. Guns still get into the country and into the hands of the Yakuza and similar organizations. In order to counter this the police or other security forces would have to be armed to counter the threats of gangs or the petty thief. There are two problems with this (I'm just going with guns for simplicity)
1). Criminals will usually have better weapons. They can access anything from a GLOCK to a fully automatic rifle. The average patrolman is only issued a 9mm. Sometimes they are issued a shotgun but, only S.W.A.T. have any real access to "heavy firepower". In order to counter this problem you have to equip the average patrolman with better guns on par with S.W.A.T. teams. Which leads to problem #2
2). With increases in firepower between the criminals and the police. The average citizen is the only one actually effected by the banning of weapons. They are no longer allowed to possess any means to defend themselves (outside of baseball bats, golf clubs, etc). Which leaves the citizens vulnerable to robbery. Police can't always be there to protect them and not everyone will have the money to hire their own security. So in the absence of police the only person left to defend the family is the family itself. A related problem to the difference in firepower between citizens and the people charged with their protection leads to abuse of power.
plato wrote...
Every care must be taken that our auxiliaries, being stronger than our citizens, may not grow too much for them and become savage beasts.
Without the means to protect themselves it leaves the citizens at the mercy of abusive police or criminals. History has proven several times that the removal of weapons from the hands of the ordinary citizen only leads to trouble.
As for the carrying a weapon on your person for protection on the street is nonsense. The only chance to use such a weapon would be if the thug turned his back to run. Which would lead to the thug just shooting his victims before he runs. In the end it is impossible to prevent humans from killing one another. It is simple human nature.