Hells greatest weapon

What happens when Hell's greatest weapon goes on a case and suddenly becomes good? What happens when a girl who is sent to kill ends up getting more then she bargained for? And what happens when she betrays her king in the worst way possible? What happens when good falls in love with evil?
Alex is Hell's greatest weapon. She is a combination of angel and demon and she has extraordinary powers that nobody can compete with. So when Crowley her king tells her to kill she naturally goes on the hunt. But what happens when she decides that she doesn't want to be like this? What happens when the angel part of her suddenly goes into overdrive and pushes and pushes until she realizes she can no longer do evil? Well what happens then...you'll just have to find out.