SPOILERS:
They didn't do a good job at all with Luke's character in this film. In the previous films, Luke sees good in Anakin and works through the whole Return of the Jedi to convince Vader to turn to the good side, and finally achieves it. Yet in this film, when he sees the dark side in Kylo, he just tries to kill him in his sleep?
Star Wars VI: Return of the Jedi - Darth Vader's Death
Here is one of the greatest scenes in any movie I have ever watched. The Last Jedi completely negates it.
They tried to not do the same thing as Obi-Wan and Anakin, where Obi-Wan refused to believe Anakin had gone bad. The prequels weren’t great, but they pretty much created the more compelling story of how a Jedi Apprentice would turn bad with Anakin Skywalker. However, going so far outside the conventions often produces a chappy narrative.