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”. Maybe this was Luke reaching a realization that the horror story of how Darth Vader was born was impossible to stop. Or even if it were possible, he couldn’t figure out why he messed up, so he figured he wasn’t the Jedi the galaxy deserved or needed.

So Luke was freaked out just as much Ben was freaked out. That event created not only Kylo Ren but also broken Luke.