Complex sentence: Ida takes up the role of detective, hunting down Pinkie in order to bring justice to Hale, and later, when she finds out that Pinkie is marrying Rose, to save the girl.
Split into simpler sentences: Ida takes up the role of the detective, hunting down Pinkie to bring justice to Hale. Although this is her original motive, when she finds out that Pinkie is marrying Rose she does so to save the girl.