Vladius said:

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.

That's fine, but at what point can we suspend our disbelief to think that Luke would actually turn to the darkside here? If one moment of weakness and anger is truly enough to cause a complete and total fall to darkness without any other motivation for our main character it makes the story feel extremely shallow to me.