In the EU, Luke becomes the reborn Sidious' apprentice. During this time did he kill any Jedi or innocents, like Anakin did, to fully turn to the dark side?
It was then that the resurrected Emperor struck. Using his dark powers to create a powerful Force storm, he swept Luke Skywalker to Byss. There, he revealed himself to Luke, and revealed the power of the dark side. Faced with an immortal foe, Skywalker did the unthinkable — in order to defeat the dark side from within, he knelt before Sidious, and submitted himself as the Emperor's new Sith apprentice