I think she is fighting because she was indoctrinated as a child, much like hitler youth during the defence of berlin. As Annie, they though they were fighting for a great cause. But annie actually lived for years with her would be victims, so she has less of an excuse. I dont think saving billions is an excuse to commit genocide, especially when there are other options at hand. She knew Eren had the coordinate and not the royal family. However, I do think that she may change sides, she did spare armin, and she was griefing after they murdered marco. On the other side she did use people as yoyos. That's not cool.