Luke's big backstory moment with Ben makes perfect sense because Luke is not the superhero paragon nostalgia tells you he is. He's a man, not a legend, and a man who makes mistakes. It's right there in RotJ - Luke is a righteous man and a compassionate man, which also means he has the capacity for righteous fury inherent in all good people. He shares the Skywalker flaw of wrath, just like when he nearly murdered the very father he was trying to save in a fit of rage for threatening his sister.
His split-second contemplation over Ben isn't a regression of his character growth or even a retread. It was a step forward! Luke didn't even attempt to follow through on his impulse beyond igniting his saber, he didn't engage his nephew in a violent duel and beat him to the floor, he didn't cut off Ben's hand. It hit him, he grappled with it, and then he mastered himself. He's come much farther in controlling his darkness than he ever had been before in the OT.
And a fundamental mistake about Luke seems to be that many fans believe he "won" his war with the dark side and never would grapple with it meaningfully again. That he was somehow above it, after sparing Vader. That's never been implied nor shown to be the case in the films. What is shown is that Luke does not give in to the dark side, not that he has beaten the dark side. The dark side isn't a fight you win: it's a boulder you roll up the hill, over and over again. You can't rid yourself forever of the temptation because the temptation isn't exterior. It exists inside you and only dies when you die.
Perhaps that is why dark-siders are so terrified of death. Only in death are we shown spirits absolutely free of the dark side (Yoda, Anakin, Obi-Wan) in the films proper.
Luke's character wasn't broken. He didn't fail his test. It was Ben, and only Ben, who was tempted by the dark and succumbed to its lure. And in the end, "nobody is ever really gone."
Luke Skywalker is shown to be a man who would indeed never go through with murder, and believes redemption is never impossible no matter how dire. He's shown as he is, not as we'd like him to be or want to pretend he was. Luke isn't a hero because he is a flawless man. He's a hero because he defies his flaws and fights with every ounce of his soul to be better than them. Luke isn't a hero because he is never tempted to do wrong. It's because he faces that temptation and experiences it like the rest of us and still chooses to do the right thing.
The people angry with Luke's character are honestly missing the lesson of the film. Because Luke feels the same way they do: he wasn't a perfect mentor, uncle, or man. Therefore he must be worthless. But Luke realizes he was wrong - that the legend of Luke Skywalker has value because it's imperfect, not the other way around.
