I guess what I was trying to say Dolche was that, given many kids go through a long and confusing phase of hating their natural parents, it must be so much harder for you, and her, to be in a step mum situation without the given that she is completely loved.
I am not casting blame, I just think for her she has had a family break up to cope with (and isnt in any control of that), and doesnt have the reassurance that kids often need that they have their home and their parents no matter what. You keep saying there is no good reason for her to feel what she does towards you, "she just does'.... I doubt its all that personal, her reason is probably that you are not her mum and you are with her dad... thats a lot of fuel for a teenager!
Her reaction when you said things might be bad enough to leave shows their is strong feeling there. (Many kids blame their parents breakups on themselves) She is obviously in a lot of pain over it all. So are you. I understand how provoking and out of line she is being.... I guess the difference is if you were her real mum and she was still at home a) this might not be happening, but also, b) this might still happen, but with the power and risk removed by the unconditional nature of the love from her parents. There is a big difference between condoning the way she is acting and understanding it. It sounds like their needs to be some understanding for there to be any change.
My friend went through a very similar thing with two step daughters, and eventually they went to family therapy together, and although my friend had little hope it would work (an example; they had stolen money from her, and tried to prove she had been flirting with their friends!) she was willing to fight for these girls, and in time it did. They are now able to talk about how they feel, and are friends. I was amazed by the change.
Im not telling you what to do, but I do think it cant get any better without you deciding thats what you are going to fight for.... and letting her know she is loved and important enough for that, if that isnt how you feel or what you want then you need to be clear about that, as things are they are just going to get worse and people will end up more hurt and angry, until eventually you can say you want nothing to do with her.
I am sorry you are having to go through this, it must be hard on your DH and your son too. I do not blame you for your anger at her, but I guess I dont blame her either. (While clearly not thinking what she is doing with those feelings is Ok for anyone). Good luck with whatever you decide to do.