First off she was a evil character (wanting to kill the heroes and all) but it was the circumstances that made her become vengful.
She was so overcome with anger that the man she loved had died that she sought revenge.
She swore revenge and that she would kill the "heroes".
She cloned/copied Rezo and the wizard Vrumagunto do her bidding.
She was never told that Shabrinigdo was reborn inside Rezo and had to be killed to keep the peace in the world.
