Klaus's reaction isn't just about the betrayal; it’s about his deep-seated paranoia being proven right. He views Rebekah’s choice as a death sentence she personally signed for him.
The episode's climax is often cited as a turning point for the Mikaelson family dynamic. [S1E14] Long Way Back From Hell
The witch Genevieve (brought back by Celeste) forces Klaus to experience Rebekah’s memories. He learns that his sister—the person he thought was his most loyal ally—summoned the one man he truly feared. Analysis: Brother vs. Sister Klaus's reaction isn't just about the betrayal; it’s