In storytelling character traits are very important in giving dimension to characters. The rule of thumb is that a character should have three to five traits. Having too few will leave it feeling one dimensional. Having too many can convolute the character, making it a nightmare to write.
One of the character traits is usually bad, dysfunctional. Greed, unfaithfulness and hate are a few examples. Moving this bad trait to a good one is called an arc. Scrooge is a textbook example of character arch. He only thought of himself and was selfish and ungiving. These are not three traits because on paper they look very similar to each other when shown. By the end of the story, Scrooge learned that the world was bigger than him, that love was important, and giving to others in need filled the soul. He has become the person he should be.
To write the sequel to Scrooge would be difficult. The writer would have to come up with another bad trait for Scrooge to have and then show how that happened. This would allow him to traverse another arc, allowing him to become the person he should be. Again.
In movie sequels this is often done. That's why they often feel false and forced. That's why a lot of sequels suck ass. This brings us to The Last Jedi.
In my humble opinion, Rian Johnson, the writer and director, had gone straight down the garage compactor. He had taken an iconic and loved character, Luke, and destroyed all of the work the initial Star Wars trilogy had accomplished.
The first two movies taught us that Vader is the baddest fucking dude in the galaxy. He kills and tortures people like a psychopath, destroys his daughter's home planet, and doesn't shy away from freezing people in carbonite. His own son picks a fight with him. But instead of giving him a slap on the hand, Vader kicks Luke's ass and chops off his hand. The masturbatory one!
Despite all of that, Luke wonders whether he can rescue his father from the dark side. He converses with his old quirky teacher, Yoda, who states, "Once you start down the dark path, forever will it dominate your destiny." In other words: Yo pops...he gone, bruh.
That was Yoda. Grand Master Jay. Short for Jedi grandmaster. And still, Luke be like, "Yo. He mah pops, sucka. I see da good in him, bruh. I'm out. Peace."
By the end of the third film, Luke rescues his father's soul, drags his body into his ship as the new Death Star falls apart all around them, and gives him a proper burial. Respect. Luke is a dude that sees past the worst of you and says, "Ya aite."
Down the garbage compacted we go.
In The Last Jedi, Luke had taken his nephew, Ben, under his wings and taught him the ways of the force. To Luke's dismay, the dark side was strong with Ben. So what does an uncle do? Kill him. Ben finds out about Luke's plan, which sends him farther down the dark path.
For Luke to look at his nephew and have no hope betrays one of his core character traits. It would be one thing if Luke found out that Vader had no good within him. Then, attributing that to Ben would make complete sense. But when Anakin shares a father/son moment right before the Death Star blew up, and said, "Tell your sister that you kissed in the mouth in Empire Strikes Back that you were right. You were right."
Johnson's version of Luke was so off putting that even actor Mark Hamill, who plays Luke, couldn't help himself but have a dead stare.
I'm not sure if Johnson wanted to give Luke an arch to travel, so he had to force a bad trait on the character. But Johnson didn't have to because there are other characters. Not every character has to have a character arch. Maybe Johnson didn't know that. Given how he torpedoed The Last Jedi, I wouldn't be surprised of his ignorance.