This interpretation is basically a blend between his Killing Joke and 2019 movie origins. He was a normal guy who was down on his luck and could barely support his pregnant wife, he overheard a couple of criminals about a job infiltrating ACE Chemicals and decided to join them. After most of the other men were gunned down by the cops during the job, Arthur fled to the catwalks where he was then pursued by Batman and in a panic, he slipped and fell into a massive vault of chemicals. Unlike in KJ though, the effects that the chemicals had on his mind and body were far more eventual than instant. His skin grew pale, his teeth became yellow, his facial structure started to be able to contort in odd ways, and his poor, poor mind was growing less and less sane. The effects of the chemicals combined with his crummy life further amplified his descent into pure madness and one night without thinking, he stood up from bed, grabbed the pistol from the safe under the bed, loaded it, looked over at his sleeping wife… and… bang.