Luke is special. He is the son of the chosen one, and the last of the LS force adepts. He isn't all powerful in the movies, but he learns to use his emotions to give him strength, yet he is able to resist the DS in him. He let loose his rage at Vader, but didn't become swallowed up by it.
Like Anakin kills the Jedi and Padme, the Emperor thought that like father like son, Luke gave in to his urges and will walk the path to the DS. It was Luke's rage that gave him the strength to defeat Vader.
But Luke is special. The force is strong with him in the end. Obi-Wan and Yoda didn't have all the time in the world to train Luke. They had to use speed teaching, have Luke kill Vader and the Emperor, and save the galaxy.What they didn't count on was Luke having the wisdom to try to help his father return to the light.