Well, spirits are not like us. They live with a purpose given at birth. And it is their duty to fulfill it. They view it as an honor, a path to follow granted by the very Gods. Maybe not all spirits share this view, but she wouldn't do it if she didn't want to. 

And I thought her motive was clear? If she fails the world will continued to be ruled by the evil lord. I also have a personal purpose for her, but I don't want to ruin the surprise. 

I guess I can change the sheet and drop hints here and there.