Yes, it requires more than just a gun to kill someone. It takes a PERSON, wielding the gun, pulling its trigger, to kill someone. If a man bashes another man's head with a hammer, killing him in the process, is it the hammer's fault?
Who's fault was Hiroshima? The bomb (the tool), the man that dropped the bomb, or the man that made the bomb? Or are all three culpable?
I would say that the culpability stops at the pilot of the Enola Gay, though Harry S. Truman, his joint chiefs, and the various military commanders between the President and the Enola Gay crew, are most culpable. Faulting an inanimate object like a nuclear weapon is silly, so is faulting the designer.
This isn't an apples-to-apples comparison, though, since most shootings in this country happen between individuals as a spontaneous act, absent the strictures of military command.