Sepharih said:

Vladius said:

That's the problem, you're oversimplifying a situation that was never simple. It's partially the Emperor/Darth Vader's influence in the Force, y'know, like Yoda and Obi Wan told Luke many times over the course of both ESB and ROTJ, and partially Luke's other motivations, which the Emperor exploits.

I'm saying that there should be more established and better developed motivations for Luke's near turn.....and you're saying that it's the emperor's power of persuasion in the force that defies rational.

There are, you're not seeing them.
You think that he's turning to the dark side when he attacks Vader, he's not. He's angry at the idea that Vader will try to turn Leia, along with everything else that happened, but that's the thing that makes him actually start attacking. It's well established already that they love each other, romantically or platonically, in all three of the movies. In the same movie, Luke has rescued Leia and showed concern for her well being. There is a scene where he tells her that they're brother and sister.

His "near turn" is within the duel after he starts attacking, when he goes from participating to becoming the aggressor. This is because he is using his anger to help him succeed, which has natural consquences that are also well established. That's why he looks surprised and disturbed with himself after cutting off Vader's hand. He's realizing that he's gone farther than he intended to.