In contrast to most depictions of The Joker, Tim Burton’s first Batman movie directly identified who The Joker was pre-transformation: Jack Napier, mob boss Carl Grissom’s second-in-command who, as was revealed later in the movie, killed Thomas and Martha Wayne decades earlier. In the ‘present day,’ Carl sent Jack to oversee a raid at Axis Chemicals, but it was all a ploy to see Jack killed for sleeping with Carl’s mistress, Alicia.
Article source: https://www.cinemablend.com/news/2481998/the-jokers-various-origin-stories-explained