If guns were banned we just see a huge spike in illegal firearms sale. So the bad guys still have guns but the good guys dont. :P
Also while yes they are more deadly you have to relize also, cars are deadly too. You could easily get into a semi and plow over groups of people and kill with much more efficency than you could with a gun. All the murders done with guns were done with the intent to kill, and sense they already had that intent and that will to kill one can only assume that they would be just as willing to kill someone with a knife or a a blunt object.
PS You know those police that anti-gun people say should be the only ones with guns, They have access to all sorts of body armour, automatic weapons, and high caliber rifles and could easily out-gun any civilian with a completely legal weapon(assuming you live in one of the more moderate states, Texas I dont know about) and the fact that they can easily radio in MORE people with the same access leads me to wonder why they didnt show up and, you know, save all those people being killed by the evil nasty legal guns.