Almost anything can be used to injure or kill someone. Does this mean that everything that doesn't have a legitimate every day use should be banned?
You're quite right, almost anything can be used as a weapon. The point is however, that guns and swords have no other use other than as a weapon, and therefore need to be controlled. Kitchen knives*, motorbikes, bricks are not dangerous weapons although they all can be used to kill people.
*I read a while back that doctors were asking the government to ban kitchen knives with points, after chefs agreed that the point was not necessary. Another sensible bit of legislation then imo - I hope it goes through.