why was Gilly being so nice to William Earnest?

As she eats the cookies, Gilly decides that she will use William in her life of crime. She begins spending time with William to get him to warm up to her. She even makes him a paper airplane that they fly outside. She compliments his paper airplane throwing skills. Mrs. Trotter looks on, as Mr. Randolph reflects on how he always thought paper airplanes were meant for annoying teachers. Mrs. Trotter compliments Gilly for her kindness, and Gilly is secretly moved, having always wanted that kind of approval, but not from someone like Mrs. Trotter.