Twilight (novel series)
When Jacob learns that Bella is "sick" at the Cullens' house, he thinks she is undergoing transformation into a vampire. He is overcome with anger, and runs to the Cullens' house, planning to kill them all. However, when he learns that Bella is pregnant with Edward's child, he goes back to La Push to tell the other werewolves. Believing that a vampire child would endanger all humans in the area, the rest of the pack plans to kill Bella and the child. However, Jacob refuses to let them. He breaks away from the pack and returns to the Cullens' house, to warn the Cullens and to help protect them. Leah and Seth Clearwater also break off from Sam's pack to join Jacob's pack.