The big moment of the film, Ben killing Han carries way to little weight because of the lack of any basis of their relationship. I felt no connection or emotion in Han's death scene other than Han is dead. Not why Ren killed him or why Solo loved him. Sure there's some exposition about their relationship but exposition doesn't give us an emotional connection, it isn't character development. It's just information. Leia was just explaining things to the audience, that was it. It would have been nice to have seen some kind of reciprocal love existing in that family - between father and son - before that final act irredeemably ripped it apart forever. If we could have seen Ben's love and admiration for his father, then his final hateful act would have carried a tragic weight. As is, it's too artificial, we know it's supposed to heartbreakingly tragic, but the biggest complaint I see from fans is that most people didn't feel it. I didn't feel it. It was just a shock to see a nostalgic hero die.
The script tried to show Kylo Rens inferiority complex, but with so many people upset that Ren is a weak villain, this message was lost on a lot. Perhaps people viewing the movie over and over again will bring to light just how important his inferiority complex is to his character. But really, my only complaint about the movie is how this important aspect about such an important character was left to just a few lines to establish. It would have been much better if perhaps when Han and Leia were talking, a flash back is shown where Ben is young and can't seem to please his father (Han), or believes he is a failure even though Han doesn't think he is. Then, in the scene where Ren kills Han, it would have been better if Ren surrendered even more than he did in the movie. Perhaps completely let go of the lightsaber... they hug... and then begin walking side by side, leading the audience to believe Han succeeded... perhaps then switch to a scene of the X-wings making progress... then go back to Ren and Han, where Ren says something like "I'm sorry I failed you" to which Han replies "Son, you never failed me", which hearing it infuriates Ren so much that in a split second the light saber is in his hand and in Han's gut. That would have done a much better job of establishing Ren's inferiority complex than him changing his mind with no dialogue. With such a weak death of Han scene, Ren just comes across as a ####, and we're left to Rey telling him that he's afraid he'll never be as powerful as Darth Vader to understand why he's such a weak villain. The lines are there explaining why he's so weak, which makes him such an interesting character. But it could have been built up better so he doesn't come across as a weak character like so many are comprehending him to be. Again, perhaps repeated viewings will help people understand how interesting he is because of his weaknesses.
He didn't change his mind imo. Just needed the strength to embrace the Dark Side (which is established in a couple prior scenes).
I'll grant that I'm not a Star Wars nerd to the degree that I've read as much EU stuff (can't believe I just typed that) as others here. So I could easily be wrong here.
But what you said goes contrary to my understanding of the dark side. Yoda told Luke that the dark side is "easier":
Luke: "Is the dark side stronger?"
Yoda: "No, no, no. Quicker, easier, more seductive."
If Ren is being torn between both sides like the scene suggests he was, he needed strength to choose his father over the dark side. He didn't need strength to embrace the dark side. He embraced the dark side in the scene because he was too weak not too.