Joseph Chilton, better known as his street-name "Joe Chill" was a hitman for hire, and often worked for Carmine Falcone. Chill was often hired for small time jobs, and he would often murder people that either tried to double-cross Falcone or were threats to the organization. Joe would also be hired by a couple of Falcone's associates, include Hamilton Hill and possibly Thomas Wayne.
Eventually, Hill contacted Chill and gave him a job: Kill Thomas and his wife Martha and make it look like a mugging gone wrong. Chill agreed and ambushed the couple as they returned from the Monarch Theater with their son, Bruce. Though Thomas pleaded with Chill to let him and his family go, he commented that "it [wasn't] personal", before he shot Thomas three times, twice in the torso, and once in the head. He then shot Martha twice, and stole her pearls whist she bled to death from her injuries. As Chill turned his gun to kill Bruce, he heard sirens and decided to leave, which spared the child of a similar fate.
Eventually, either due to evidence that linked him to the murder or he was being caught while he fled, Chill was arrested and sent to Blackgate Prison. His charges may have included other assassinations that he had carried out for Falcone and Hill. While in prison, Chill was at attacked and stabbed to death by another prisoner or a guard on either Falcone or Hill's payroll.
Chill's murder of Thomas and Martha lead their son Bruce to become the vigilante Batman. It also lead to many of Gotham City's great families to "flock" together, and believed that their fate could have happened to them. Bruce later found out about the connections between Chill, Falcone, and Hill after news of his parents' involvement with the city's corrupt was uncovered. When he later investigated Crime Alley and recounted his parents' murder, Bruce considered what he would have done if he had the chance to meet Chill again, and chose between arresting him, murdering him, or demanding to know why he had killed them.