It could be argued that Miller was simply hearkening back to the early incarnations of the character, but you shouldn't make that argument, because then you are siding with a racist nutjob. Still, it is technically true: Batman was originally inspired by characters like The Phantom and The Shadow, who appeared in pulp magazines and radio serials and (along with being billionaire playboys with mysterious alter egos for fighting crime) frequently dealt with their adversaries permanently. As in, they shot them. They shot them to death. Or else tossed them off cliffs, or beat them to death with their bare hands. All of which are fates which befell early Batman villains. In fact many of the enemies which ended up becoming classic members of the rogues gallery - including no less than The Joker - were supposed to be killed off in their first appearance, and sometimes were, only to be unconvincingly resurrected a few issues later. In a few early stories the Caped Crusader ran criminal's cars off of cliffs, shoved a dude onto a sword, and hung a henchman from a noose attached to the Bat-Plane. "He's probably better off this way," the superhero opined off the dead man attached to his plane.
Tom Baker is the Comics Editor at WhatCulture! He's heard all the Doctor Who jokes, but not many about Randall and Hopkirk. He also blogs at http://communibearsilostate.wordpress.com/