Another thought since DrDre mentioned it: I also was having issue with how Ben Solo’s only ‘crime’ was having the taint of the dark side in him. But perhaps one could think that Luke is a perfectionist in the prime of his career. Maybe he couldn’t digest it well or fathom that one of his own students could already be lost to the dark side. He simply wasn’t prepared to deal with someone else, a pupil, turning. In that moment, Luke was seeing that future of Kylo Ren being born and causing much anguish. It’s not like his previous Force vision in ESB turned out to be wrong, as yes, his friends were in pain. To Luke at that point basically learned the future is set in stone, and not like how Yoda said it where the future is “always in motion”.
So Luke was freaked out just as much Ben was freaked out. That event created not only Kylo Ren but also broken Luke.
The point is, I can understand Luke feeling the tremendous burden and responsibility of restoring the Jedi order on his own. I can understand Luke getting upset after seeing visions of his Jedi order destroyed, and trying to figure out what to do about this. If there had been some buildup to the scene we witnessed, it might have been more easy to swallow, for example if Ben had hurt one of the students during training, and seemed to enjoy the experience. This would make Luke’s reaction, while still being distraught about what he had just witnessed, understandable. As it is now, the film says Luke saw some dark thoughts and a possible future, and couldn’t control his fear, as a fully trained Jedi should.