Question

What does macbeth claim is his only motivation for the assassination?

Answer 1

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.