Klaus himself had killed thousands of times, and yet he could not become a werewolf, because his mother had prevented it. She had cast a spell to cut off that part of him, locked it away forever and called it “balance.” Her magic honored nature, except when her pride or infidelity perverted it. Because of Esther’s hypocrisy, this was a path down which he could not follow Vivianne, should she choose to go.

- The Originals: The Rise, Chapter 10