During the subsequent lightsaber duel, Luke succumbs to his anger and brutally overpowers Vader, but controls himself at the last minute; realizing that he is about to suffer his father's fate, he spares Vader's life and proudly declares his allegiance to the Jedi.
During the subsequent lightsaber duel, Vader learns that Leia is his daughter, but when he threatens to convert her to the dark side, Luke succumbs to his anger and brutally overpowers Vader. However, Luke controls himself at the last minute; realizing that he is about to suffer his father's fate, he spares Vader's life and proudly declares his allegiance to the Jedi.