It is his fault that he lies by omission. If he wants to do a deal he should tell them what they are getting into. They may still choose to do the deal or choose not to, but at least then it wouldn’t be so evil of a thing. Like tricking a woman for her first born child just so that she could get out of poverty is pretty evil, but if he told her up front maybe she would have realized she could change her life on her own and just do it. I know it wouldn’t be as good of a story, but that doesn’t change the fact that what Rumpel is doing is lying and is causing pain and suffering and that he doesn’t care about causing pain and suffering as long as he is able to get what he wants.
I think that if we really go back and think about what we know all of the evil that is happening that we have seen has been caused by Rumpel. Even what happened to Regina because i think that Cora did a deal with Rumpel to get her power and that led to Regina becoming who she is and that lead to Regina gaining magics as well so the long consequences of Rumpel’s choices have are horrifying for the world. That doesn’t negate the choices of the people who did deals with him, they are still in the wrong, and certainly Cora and Regina are in the wrong, but the choices made by those people also do not negate the choices Rumpel makes to do evil and the fact that he is under the dark one curse is not enough to say he is not culpable. He knows what he is doing and based on Skin Deep he can make other choices, but he doesn’t.