How is Pip a victim of his own ambition?
In Great Expectations, the theme of ambition does not necessarily have a negative connotation. You need to think about how Pip's ambition was aroused in the first place. As a very young child, he was introduced to the ways of the wealthy and sophisticated. He found it very hard to be mocked at because of his coarseness, and longed to become a gentleman in order to be accepted as an equal in an environment corrupted by haughtiness and extreme moral suffering.
You could perhaps view him as a victim of his own ambition at the moment when he discovers that he owes his advancement in life to Abel Magwitch. He has deluded himself into believing that Miss Havisham provided the means to turn him into a gentleman so that he and Estella could marry. It was extremely humiliating for him to find out that his benefactor was a convict, someone to whom he could not be connected now that he had risen in good society.
However, these feelings were short-lived, as his natural kindness prompted him to help Magwitch at the risk of losing everything. Thus you need to ponder this question very carefully, since it might lead you to misconstrue the character.