Spoilers below
Keira's grandfather is. Technically Cosette is just a demon fulfilling its contract. That's just its job. Also fulfilling the contract would help its demon race. And Cosette didn't kill Keira, the kingdom did. They could have simply kept her under surveillance as she lived out her days.
The grandfather KILLED an innocent woman and her baby simply for power. He was already rich at this point. It wasn't even for survival. It was greed. Which to me is much more evil than anything Cosette ever did.
Before time turned back, after Keira's death, Cosette/demon even says to the Duke what they did to his ex wife was heartbreaking.
OMG if the demon had a handsome male form. And duchess Edinburgh ends up falling in love with him. And instead of taking her soul he takes her as his bride, She gets a second chance at life and love. Now that's a happy ending.