Complex sentence: The Joker explains that years ago, he was a lab worker who decided to retire and steal from the Monarch Playing Card Company but after being thwarted by Batman, he jumped into the chemical waste to escape.
Split into simpler sentences: The Joker explains that years ago, he was a lab worker who decided to steal from the Monarch Playing Card Company. After being thwarted by Batman, however, he jumped into the chemical waste to escape.