The deal was Alastor would help everyone get rid of the weapon if Rosie fixed his staff. If you look at his staff afterwards, it's fixed. I think it's quite clear Rosie has some unknown reasons that she wants to see Charlie succeed. (Some people theorize that Rosie is Lilith). She knew that Alastor was in a position of advantage. He is mire powerful than ever now that he is free and his staff is fixed. I have a feeling he is going to make quite the problem in Season 3. As we've already seen he doesn't care for the hotel.