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.
The passage describes the Author's impression of students participating in the "Upward Bound" program and it is generally favourable towards the program, without giving a more specific reason for this favourable opinion, it's more the author's personal impression.
Thus, we could say that the author has a positive bias towards the program.
