jedibuttercup

Luke's turn to the Dark Side actually happens between the original EU trilogy (the Hand of Thrawn trilogy by Timothy Zahn) and the Jedi Academy trilogy, which are 5 and 7 years after ROTJ, respectively. The Jedi Academy trilogy mentions it clearly, which confused me, too; I thought, when did this happen?

Turns out the Star Wars comics are considered as canon for the EU as the books are, and a "Dark Empire" and "Dark Empire II" series happened between the trilogies, set 6 years after ROTJ. In it, we discover that the Emperor had, in fact, managed to cheat death; he had clones waiting on a world rich in the Dark Side, and once free of his body after ROTJ managed to reach out and possess one of them. He lures Luke in by promising him knowledge; he has a Jedi holocron chock-full of data, and Luke is desperate for information on how to restart the Jedi Order.

He doesn't kill anyone in person that I remember while serving as the latest Sith apprentice; he keeps thinking that maybe he can prevent some atrocities from happening. Good intentions, and all that. But he does eventually command a battlegroup attacking the Mon Calimari homeworld. Han and Leia fly in on a desperate rescue and manage to talk him down a la Luke vs. Vader, but not before he almost kills Leia, who is pregnant with her third child at the time. Tense sequence, all 'round. After he snaps out of it, he kills all the clones he can find, hoping the Emperor won't manage to come back yet again.

It haunts him, in the books, and leads him to realize that using the Force for everything isn't such a good idea. (A la Obi-Wan telling Anakin not to use it to float fruit!) Power corrupts. This leads to him taking the title of Jedi Master - perhaps the whole event served as some sort of trial for him? - retiring from active fighting work for the republic and setting up his Jedi Academy.