Mortus would not be able to live with herself if she unleased a demon on the world. I know it isn't her fault but she wouldn't see it that way. She is worried enough about magic being evil and getting burnt alive, can you imagine what will happen to her if she is accused of demon worship? Not even Max could talk her out of that one. So fireball it is, hope it doesn't also kill the doctor, hope he is not dead and then figure out what the hell is going on.
Also if this doesn't work she doesn't really have a plan B, though Mortus doesn't think she can out run a demon.