Stockholm syndrome is a fair point, and that is likely part of the picture, but I don't think that's all. Since Ram was deluded by her village's worship as the strongest demon, and pushed into that role with no say from her, when she noticed that Roswaal was obsessive with his goal and so attached to the past that was imprinted upon him, at some point, I think she wanted to break him free of it, because she realized that she was glad to escape from her horn and the expectations it brought, as well as the negative, violent emotions it brought upon her when using its power sometimes (ex: when Rem lost control in the forest using her demon form). Now, since Ram has been slowly having mana put into her horn by Roswaal for quite some time, she can use part of its power, but only for a limited time, and not without a great cost, of course.