Sid was an incompetent and bumbling little man who always seemed to screw up all his attempts at crimes. Sid had always wanted to be a big shot in the mob and a "friend," Eddie G., let Sid go along on a job as a lookout. However, he was only brought along as a decoy for Batman. While the others slipped away, the Caped Crusader cornered Sid on a rooftop and tried to pry some information out of him. Sid, in his usual clumsy fashion, slipped on his dropped flashlight, causing him to fall into Batman. The other mobsters, watching their exaggerated shadows from afar, believe that Sidney is fighting Batman, saying "little weasel's got guts". Sid managed to escape from Batman's grasp, but almost fell from the building. When Batman tried to save him, Sidney unintentionally let him slip over the side of a building and into the massive explosion of a propane tank below. Sid's accomplices, witnessing the event from afar, deemed him "the man who killed Batman."
For a time, Sidney enjoyed his title and newfound fame. Many criminals considered him the "Toughest Guy in Gotham." However, with this title came challengers who sought the title for themselves. Sid, in reality a weak man, sought to evade these challengers, leading to a bar fight between his friends and his challengers which got him arrested. When he was in jail Harleen Quinzel bailed him out and took him to the Joker.
Angry with the fact that a small time crook took the honor of killing Batman from him, Joker tried to kill Sid by locking him in a coffin and dropping it into a vat of acid. However, Sid was rescued when his coffin was swept out to sea and lodged open.
Now fearful of the Joker and challengers, Sid sought out the aid of Rupert Thorne in getting out of the city. However, Rupert believed Sid to be playing dumb so that he could take over his operations himself. Fortunately, Batman, who had managed to escape the explosion arrived and saved Sid. Still, Sid was an accomplice to a drug racket and had to go to jail, but Batman notes that under the circumstances, he probably would find the situation quite bearable. In jail, Sid finally realized what the superhero meant as his dream of being a big shot comes true with all the crooks calling him "The man who almost killed Batman and made a fool of the Joker and set up Rupert Thorne." Presumably he is still incarcerated.