Why did the Volturi kill Iriana in Breaking Dawn?

In the Twilight series, the Volturi are a powerful coven of vampires who enforce the laws and customs of the vampire world. In the fourth book, Breaking Dawn, the Volturi learn that a vampire named Irina has falsely accused the Cullens, a rival vampire coven, of creating an immortal child, which is strictly forbidden by vampire law. The Volturi punish Irina for her false accusation by killing her.