I see what you're saying about guns being
designed to kill, but they can serve other purposes as well. Have a lock you can't get past and you left your birthright lock-picking set at home? Fear not if you have a simple handgun! Blow the lock off!
But what the object was designed for does not matter. What does matter is how many deaths are caused by that object. If I trip on a vine on a sidewalk, bump my head, and get a concussion, should I sue the manufacturers of the vine and demand that all vines be banned to prevent further injuries to innocent people?
You're right about the black market as well, but I will quote one of my favorite phrases:
Outlaw guns and only outlaws will have guns.
The black market will still exist no matter what. The only way to counter the outlaws is to fight fire with fire--or, in this case, defend yourself against a gun with a gun.