Answer:
Throughout the late 1940's and 1950's, MGM became the king of the movie musical. And this was largely due to the "Freed Unit." Freed's productions became so popular that the studio could afford to have a SECOND unit for musicals under the direction of Joe Pasternak.
He claims that vaulting ambition is his only motivation for the assassination.
Although he was ambitious before he talked to the witches, after hearing the prophecy (and after his wife goaded him into it), he decided to kill the king and become the next ruler himself. After that, both his paranoia and ambition only grew stronger and led him to his demise.
