A “unification” between dark and light sounds equally disturbing. It isn’t like there are two extreme factions each bad in their own way. One is good and the other is bad. We see the First Order commit mass genocide on multiple occasions. We see the Resistance save the day on multiple occasions. Even if Kylo decides to merge with the Resistance, it will appear as if the First Order won, because the Resistance is already perfect and all they can do now is corrupt it.
Now that I’m thinkin about this, I don’t know if a solid conclusion to Episode 9 is even possible with the hole RJ dug us into.
This is why Rey having feelings for Ben and the whole “Reylo” thing is utterly stupid. He is the head of an organisation that has murdered billions of people. He has personally executed people and killed his own fellow jedi apprentices, even killed his own father in cold blood right in front of Rey. She constantly spits “monster!” in his face but suddenly starts to sympathise with him and have implications of sexual attraction?
The only way it could have made sense and actually been something new, interesting and different was if she actually did join Kylo in TLJ.
Edit: Damnit! I knew posting a reply in the first place was a bad idea. I’m out for now.