I think that we will find that following the Battle of Endor, Luke has come to the conclusion that the Force is neither positive or negative, but rather seeks to balance itself out naturally. The inherent conflict between the light and dark is the result of a natural attempt to regain it's balance. In the interest of universal harmony, Luke, Leia and the others have attempted to conceal all knowledge of the Force for fear that an attempt to use only the Light automatically gives rise to a Dark force to counteract and restore balance or vice versa. This is also why Luke is missing from the posters. He has decided to conceal himself and stop using the Force, in the same way that his Masters Obi Wan and Yoda did for fear that his own power will only give rise to another Dark Lord. The only problem with Luke and the others attempting to make the Force a secret is that this tends to result in those that want to find out what that secret is, enter Rey, Finn, and Kylo Ren. Kylo Ren is attempting to pick up the mantle of Anakin Skywalker who was to bring about balance with the Force. In the absence of a true Master, Kylo Ren is left attempting to piece things together from clues left behind resulting in imperfect knowledge of the Force. This manifests itself in things such as his lightsaber and need for crossguards. Anyway, we will see soon enough.