As I see it a Padawan has the option to go either way during their learning, they can go the way of the Jedi or be tempted by the power of the Sith. In the older times, a padawan has a mentor, someone to walk them through the journey and keep them in line. Luke had none of that. He was learning on his own with the ghost of Kenobi giving him tips and a real short time with Yoda. In theory it left him open more than anyone else to see the power of the Sith, the temptation of the dark side and embrace it, even if it were just to rid the galaxy of the Empire (we've seen that in the Anti-Hero model over and over in various other franchises). Even in his bleakest moments, he avoided it, cracking temporarily in his burst of anger against Vader but decides the light side of the Force instead, ridding himself once and for all of that temptation to which the Emperor attempts to kill him for the shunning and refusal.
By the end of ROTJ he is a Knight and has gone through the trials. To have a cloned Emperor return and have Luke be tempted again puts him back three steps and destroys the feeling that Luke despite not having the same benefit of the Knights of the Old Republic overcame the flirting with the dark side to be a Jedi in his own right. Dark Empire doesn't strengthen that IMHO it makes the end of ROTJ watered down because it makes it less powerful, less emotional and less relevant to the journey of Luke Skywalker himself. The original mold for Jedis were like Monks. They achieve oneness with the force through peace. Act only when prompted and never out of anger. I saw that in the end of ROTJ before Lucas changed them to feeble space cops.
That is of course removing the Vader aspect because when I was a kid up until 1999, Star Wars was about Luke Skywalker....
Didn't anyone watch ROTJ? Seriously.
I thought that might be it. Luke isn't tempted by the dark side. Go back and re-read those comics.
Here is a cliffnotes version.
Luke discovers the emperor is still alive.
He has a holy crap moment, and decides to try a gamble, and Lies to the emperor.Luke lies about becoming the emperor's servant and replacing his father.
He does this to get close to palpatine, discover HOW he came back, and find a way to stop it.
While executing his plan, luke is forced to do terrible things inorder to "maintain his cover".
The emperor knows he's bluffing, but relies on the seductive nature of the dark side, and the acts luke is being forced to commit to turn him.
As the ends do not justify the means, all that time spent in a den of evil starts to have an effect on luke, and he Slowly starts to fall to the dark side over a period of time. He doesn't even realize it.
In the end, Leia Faces luke and brings him to his senses, and together they turn on the emperor.
the "jedi moral" of the story?
"the dark side is ALWAYS there. just because you outwardly reject it once, it can still creep into you like a disease if you're not careful. Constant vigilance is the only way to keep it at bay. Anyone, at any time may slip. "
Just because Luke Rejects the dark side and becomes a knight, he's not immune to the dark side. anakin was a knight, he became a sith lord. Same with Dooku.