Luke's entire inner conflict in ROTJ is that he has now discovered Vader is his father and that has stirred a deep emotional conflict within him, a conflict that wasn’t present when he first faced Vader in ESB. Then, he faced Vader looking for retribution because he believed Vader killed his father. Luke also realizes that this was the wrong reason to face Vader and thus ensured his defeat. Sure Luke went out to try and save his friends, but it’s that attachment and his desire for revenge that ultimately leads to his defeat. It’s during this conflict and that Luke comes closest to turning to the dark side, because he has yet to learn how to control his anger and put his attachment in the proper context.
Now Luke knows the whole truth. Luke knows what great hero Anakin once was and wants to see his father redeemed, not destroyed. This emotional conflict is also present for Vader as well and I can’t help thinking that in his duel in ROTJ he might have been holding back, truly not wanting to destroy Luke.
I know almost everyone will disagree with this, but I really don’t think look turns to the dark side at all in ROTJ. To this point Luke has learned some tough lessons. He’s seen how rash action dominated by selfish emotions have actually hurt his friends. He has learned to put his attachments in the proper context, yet he has not abandoned them. They have now become his motivation and strength rather than a weakness to be exploited.
So, In the moment Vader threatens Leia I think Luke realizes that all his attempts at diplomacy with Vader have been exhausted, and it’s in this moment he realizes he will have to fight to protect the ones he loves—his first stand against the dark side will begin with confronting his father. I think his battle cry was an emotional release of all this frustration and an expression that he will not go down without a fight. He is finally taking a stand in the right place, at the right time for the right reasons. It’s in this moment I think that he truly becomes a Jedi. This is his test—his trials. Therefore it’s not anger or hate that motivates his attack; it is actually duty and ultimately love.
Anger in and of itself isn’t necessarily bad, it’s when one give oneself over to anger to fulfill selfish desires that it becomes a negative emotion. Ultimately the dark side is about the acquisition of power over another. It is a selfish pursuit and anathema to what the Jedi stand for as public servants and protectors of peace. Luke’s motivation here is not revenge, but a desire to redeem his father and protect those he cares for.
Luke’s reaction after he cuts off Vader’s hand is one of surprise at his own power. It is also the fulfillment of the faith he put in the Force to see him through. He now truly believes the light side is stronger, that victory is attainable and he will be the instrument the Force uses to restore his father. Look at the confidence he has afterword. He doesn’t care if Palpatine kills him of not. The internal struggle is over and he’s won. He is a Jedi like his father before him. He is a man that stands for justice and peace and will not be given over to his baser emotions and desires. Furthermore, he has passed the test of temptation that his father failed, therefore becoming more powerful than him. Even if Palpatine kills him, he will die a Jedi—not a Sith. The shroud of the dark side has been lifted and the momentum of the moment is lending more and more power to the light side. He is capable of destroying the emperor and Palpatine knows it.
Then, he demonstrates this new power and maturity by taking an incredible risk. He disarms himself and takes a moral stand. So strong is his faith in his father’s love and the light side of the Force for him that he’s willing to risk his life to prove it. Love motivated him to protect those he loved through the measured application of force. He is now challenging his father to do the same—to take the stand he should have taken many years before.
None of these actions are those of a person who even momentarily turns to the dark side. They are measured and calculated. Everything Luke does is in service to someone else—to someone he loves. At no point does he seek power or revenge. His attachments have given him the strength and will to protect those he loves and restore justice. Its Luke’s love and faith that bring his father back from the dark side to destroy the emperor.