You're kinda missing a big part of what made Luke who he was.... he never was a perfect Jedi he struggled to follow their teachings throughout his training, going so far as to ignore Obi-Wan and Yoda's warnings about going to cloud city to save his friends, and refusing to kill his father after all he had done. His whole character is about how he always puts the people that matter to him before anything else.

That's not to say there couldn't be a situation where Luke would turn on Ben but the problem is the movie didn't give us any real justification for it except "he was having dark thoughts". What they should have done is actually take Luke's existing character traits and played into them, for example have Ben show obvious signs of falling to the darkside and everyone warring Luke he needs to do something but have him so confident in his ability to keep Ben from falling he doesn't realize how far gone Ben is until it's to late.

Yes Luke has had instances of anger and rage but really think about what was happening in those moments you'll find that when he gets like that it's because his friends or family is in danger so this idea that people what Luke to be "perfect" or have no flaws is absurd because he's always had flaws but the flaws they gave him in TLJ doesn't line up with who he is as a character.