We don’t even know what he saw. It could have been so bad it was clear there was no saving Ben. Maybe it was a likely future. And he had a moment of weakness, and then he came through it. If you don’t think he can have a one second lapse in judgement, then yes, you think he should be perfect.
He says he saw the destruction of everything he knew and loved. After 25 years of building and rebuilding the Jedi and the New Republic and reaching that utopia we all imagined happened after ROTJ, it’s easy to understand why he’d be so worried about it all being lost to the dark side once again. He saw killing Ben as a quick and easy solution to that potential disaster (and remember Ben wasn’t totally innocent at this point, he was already under the influence of Snoke so it wasn’t just a vision).
The quick and easy solution is obviously the way of the dark side though, but Luke will always have a bit of the dark side in him, the struggle isn’t just resisting it once and you’re all done. It’s a constant struggle, and in this moment he came just a little too close to not resisting it, and it cost him dearly. I find it a very compelling and human story for a character who should be very compelling and human and yes, imperfect.
Again ROTJ sets up Luke losing his cool by letting the viewer experience Luke’s struggles, as he witnesses his friends and allies getting killed. TLJ doesn’t do this. It doesn’t even try to show the threat of young Ben Solo, and how Luke came to fear this young boy. Had we witnessed Luke’s vision or had we been given some set up, where we see one of Luke’s student’s get hurt as Kylo gives in to the dark side, we might have better understood Luke’s reaction. However, the movie appears to be mostly interested in showing Luke’s failure in the face of some scary vision, rather than a proper setup.
The fact of the matter is, the story of this film is not about Why Luke Almost Killed Ben. That’s just what happened long ago that put Luke in the state he is now.
Yes, but considering our history with the character of Luke, and the enormous effect this bit of history has on the development of both Luke and Ben Solo, I think the movie owed us a bit more than one itty bitty scene. If I had to choose between seeing a pointless fight between Finn and Phasma, or a proper setup, further character development, and motivation for Luke and Ben Solo, I would definitely choose the latter.
First of all, I think a vision inside of a flashback would have been a little weird. Second, we see what Ben is capable of on the dark side in the films themselves. We see him burn down Luke’s temple, we see him ally with the First Order, we see him burn down the Jakku village, kill Lor San Tekka, kill Han, etc. We know the terrible power of Kylo Ren. When Luke says he sees that power, I think we get the picture well enough.