It’s also frustrating that people say Luke being anything less than perfect (i.e. one moment of weakness that quickly passes) is “out of character.” Did he not have a moment of weakness in ROTJ as well?
I also thought the fact that Luke wears white in SW, grey in ESB, and black in ROTJ was a thing that was indicative of his character getting progressively closer to the turning to the Dark Side, until he defies the Emperor at the final moment—after beating the crap out of Vader in a moment of rage. That compulsion he feels toward the Dark Side isn’t just going to go away now that he’s a Jedi.
He reacted to Ben Solo in a moment of fear and rage, same as he did when Vader threatened Leia.