With my heart ticking like a bomb in a birdcage

Kitty is involved in some pretty questionable activities in book II, and the Resistance are not nearly as noble as she originally believes. As the book goes on she becomes disillusioned with the notion of violent resistance, mainly because she’s seen Pennyfeather’s naked greed close up in the tomb of Gladstone. The magicians would of course argue that the Resistance are simple terrorists, and by the end of Golem’s Eye Kitty has realised that she has to find a different way to make a difference - one that doesn’t morally compromise her. You’re right that she’s stubborn, but she always retains her idealism that she can make a better world. That makes her a Resistance fighter to begin with, but she is not corrupted - her idealism survives intact and is flexible enough to change her course.