Anytime this arguement comes up and people think that banning guns will stop people from going to work and snapping and killing a bunch of people I always think of that one episode of the Simpsons. The Monkey Paw one:
Lisa wishes for world peace and she gets it: all weapons disappear then aliens (armed with only a 2x4) come and take over the world because we have no weapons to stop them with. Then one day someone goes after one of the aliens with a 2x4 with a NAIL in it. The aliens run off all scared of the new super weapon that the humans created.
Granted it's very far fetched and a cartoon but there is a very true underlying point to it. No matter what you do we as humans WILL KILL EACH OTHER. Sad but true it's just our nature. Take away guns and people will just use knives, take them away and it will be rocks and so on and so on. Maybe a bunch of rocks can't kill as quickly as a gun but don't worry because we as destructive humans will find a way to mod up a rock cannon so we can kill more efficiently.