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.
The movie should have let us know then. It’s not enough to keep us guessing. If the film wants the the vast majority of viewers to accept Luke’s u-turn, it needs to make us understand what causes Luke to loose his composure like that.