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.