It's become a cliché way to make sure a main character has the freedom/autonomy to do what they need/want to do in a story - kill off the parents. But doing that creates all sorts of legal ramifications for the protagonisht which must be considered. Here's a great article by Donna Ballman, author of THE WRITER'S GUIDE TO THE COURTROOM: So You Killed Off the Parents in Your Novel? It covers divorce, custody, inheritance, siblings, school, dating, injuries, and work. Great!