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 his contact, Eddie G., let him go along on a job as a lookout. However, he was only brought along as a decoy for Batman. While the others slipped away, Batman cornered Sid on a rooftop and tried to pry some information out of him. Sid, in his usual clumsy fashion, 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 them, 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.
In order to make sure Batman was really gone, the Joker brought him along for a jewel heist. However, it worked too easily, and the Joker felt depressed that Batman wouldn't be around to make it more interesting. With his most valuable source of amusement dead, Joker decides to hold a funeral in the chemical plant where the Joker was first created, with only six attending members. The coffin holds Batman's cape and cowl, to which Joker attaches a "Kick me" sign. As Joker delivers a eulogy describing the death of the Joker's dream to kill Batman, wishing his jokes could have outsmarted his cold, equanimous logic, he denounces Sidney for doing it first. When he is done, the two hoods place Sidney in the coffin, and the coffin is sent on a conveyor belt towards a vat of acid, while Harley plays "Amazing Grace" on a kazoo. The coffin then dropped into the 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 after being told by Eddie G. that he was behind the drug business. Sid told Thorne of his story revolving around what had happened. However, Thorne believed Sid was only playing dumb so that he could take over his operations himself. Fortunately, Batman, who had managed to escape the explosion and save Sid from the acid, arrived and stopped Thorne. Afterwards, Batman pointed out that Sid was still technically a criminal, due to being an accomplice in a drug racket, and as a result, had to go to jail. But he also added that under the right circumstances, a man like Sid could still find the situation quite bearable.
Sid was then sent to Stonegate Penitentiary, where he finally realized his dream of being a big shot. All the inmates cheered him on by calling him the man who almost killed Batman and, in the process, managed to both make a fool of the Joker and set up Rupert Thorne. Sidney was glad to be "a big shot at last."
*In I Am the Night, Batman mentions to Robin that he is mortal and someone may kill him; "It could be Joker, or Two-Face, or maybe some two-bit punk who gets lucky". That may have been an indirect reference to Sid Debris.