I need some advice on my story...help please?

im writing a very dark themed fantasy story, but dont get the wrong idea-its not at all like twilight, no stereotypical books like that. no vampires! but anyways, im stuck on my storyline...my antagonist is a group who is hunting my main character, who is a girl. this supernatural group is controlled by this guy...but i havent come up with why they want her so bad, why they want her dead. ive tried brainstorming and everything to have my own solid idea but it just hasnt come to me. so far i have the whole storyline of whats going on surrounding the girl, ttt nothing with how that connects her to it really...any advice on what i should do? i know i should develop my own ideas, but what do you think(could help me)?