This theory is grade-A baloney. The original trilogy deals with Luke Skywalker’s character arc: he starts out as nobody (just a kid), he then learns he has a gift, and finally learns his father is Vader. In the third act of the trilogy he does allow the darkness in, but the whole point of the final act of Return of the Jedi is that Luke ultimately renounces the dark side. He throws his lightsaber away and tells the emperor that he would never join him — he’d rather die. The emperor replies, “So be it, Jedi” and proceeds to electrocute Luke to death. Of course, Vader sees his son’s sacrifice and redeems himself by destroying the emperor.
Luke might have briefly dipped his toe in the dark side, but he and Vader both sought the light at the end. It would make no sense for Luke’s character to suddenly decide — when the huge war was over — that he would live his life on the evil side. He basically told the emperor to shove it, so why would he suddenly (on his own accord without the emperor’s manipulation) decide to become evil?
I call shenanigans on your theory.