A story must be complete. That means every villain is defeated in some way. That doesn’t mean the villain necessarily has to die, but that they lose in some way. In action thrillers like “Die hard,” it’s easy to see how every terrorist needs to die. If a terrorist survives, then that leaves the story incomplete and less satisfying. By making sure every terrorist dies, “Die Hard” creates a complete and emotionally satisfying ending.
Just remember that villains don’t need to die. Sometimes they just need to lose. In “Star Wars,” Darth Vader gets knocked around in his TIE fighter but he survives. In “Titanic,” the villain is still alive after the Titanic sinks, but he’s defeated in not being able to marry the hero. In a horror film like “Don’t Breathe,” the villain is still alive, but has failed in his goal of getting a replacement child to make up for the child who got killed.
Villains must be defeated in their goal somehow. When a villain isn’t defeated, the ending feels incomplete. In “The 5th Wave,” the hero rescues her little brother from the villain, yet the villain has no clue who the little brother or the hero is so rescuing the little brother has no impact on the villain’s goal. In fact, the villain is completely unhurt by the hero rescuing her little brother and is completely untouched by the hero in the end. This would be like Darth Vader never being defeated in “Star Wars.” It just creates an empty, meaningless conclusion.
Always make sure your villain and all of the villain’s henchmen get defeated somehow. That might mean physical death but more often, it just means that the villain fails to achieve his or her goal. In “Little Miss Sunshine,” one villain is a mean beauty pageant organizer who wants to keep the hero from competing because she signed up too late. This villain gets defeated when someone else helps the hero sign up for the pageant, and then gets defeated again when the hero finally competes and embarrasses the villain in the process.
List your villains and make sure all of them get defeated. That’s the surest route to creating a satisfying ending in any screenplay.