Anakin/Dathvader is the most powerful sith of them all said the phrothecy so he could kill the emperour easily the resons he dint:
Episode 3, he dint want to be by himself plus he needed palpetine to get the galactic empire organized
Episode 4, He wants Luke with him to defeat the emperor father and son.
Episode 5, same as 4 but he wants luke to become a Darksider
Episode 6, He relizes the true meaning of the Darkside and that luke would never join him so he decides to kill the emperor to help his son and to put a end to the Darkside.