I dont have a theory...all i know is her father was a total jerk and he really effected annie 由 saying only your dad will be 由 your side...i really hate him cuz he is the real reason that annie is like that=(
I think when Annie was young, he injected her with the "titan sent" and taught her to control her abilities, then taught her martial arts while teaching her his messed up way of thinking that made her really turn on humanity, then sent her to training at the camp(I don't remember what it's called) but before that, taught her to think only of herself and the 日 after she turned 16 (or whatever current age she is)and so she choose the military police because she should only think of herself.
