Spoiler: Kylo Thoughts, VERY spoiler
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Here's what we know about Ben's upbringing.
Raised by 2 heroes of the Republic. People always telling stories about what they did, how awesome they were. His grandfather's a legendary badass (horrible person, but strong, and people respected his power) who has similar larger than life stories everywhere.
He's strong in the force. Really strong. Combined with the stories about his parents and grandfather many people likely expect him to do great things.
He was sent to his Uncle Luke to be trained in the force. But particularly with Anakin's fall, Luke fears the influence of the Dark Side.
So he's being told to turn away from the power he sense when he feels like people around him are expecting him to do great and strong things, like he needs to be STRONG, it's his birthright, he comes from a line of LEGENDS. And now he's being told to turn away, to hide his frustration.
He's also being influenced by Snoke. My impression is that influence takes the form of stoking his frustrations at being "held back from the power that's his to claim."
Then he wakes to Luke standing over him, lightsaber drawn. His parents shipped him off to his Uncle, his Uncle has turned on him, he's living in the shadow of his Grandfather's strength
and fearful respect. He has nothing. He lashes out at Luke, and burns the place down.
So now he goes off to Snoke, all good, right? Well, Snoke doesn't really go in for the comforting thing. Highlighting his mistakes, his wavering control. Even in this new life Kylo Ren doesn't have the respect he seeks. The First Order crew is laughing at him, Snoke's belittling him, he WILL be strong, he WILL be feared, he WILL prove his strength to the parents he feels abandoned him!
When he once again meets his father, he expected to be angry. To show how strong he had become. But instead he sees his dad. He sees the family he thought had abandoned him,
reaching out. and when the killing blow comes, it doesn't leave him with the feeling of victory he expected.
Snoke, of course, belittles this "weakness." This display of emotion outside of being a cold killing machine. Riding on the coattails of his grandfather's reputation, trying to look cool in "that stupid mask." And Kylo lashes out in response, destroying the link he had made to his grandfather, the symbol of who he was "supposed" to be.
Angry, lost, he's connected by the force to Rey. She has power. She's been abandoned. Her reaction when he shares the story of Luke shows that she understands the feeling of betrayal.And the Force has linked them, surely there's a reason.
And then it turns out Snoke was using him, using his suppressed desire to belong, to draw Rey to him. Didn't even have the courtesy to fill Kylo in, just considered him a pawn to throw around. He turns the lightsaber to his true enemy.
And then, after the battle, he tells Rey what he knows. She was abandoned by her parents like he felt abandoned by his. He still feels the shadow of his relatives, his past, the expectations and pressures he put on himself due to that. But he's done Rey a favor. She doesn't have that weight. She's strong on her own merits, not due to heritage or destiny. Both split from their family, they have each other. Together, they can rule the galaxy, she just needs to join him, and he'll finally have someone who respects him.
And then she declines, and Kylo falls back into violent lashing out. Needs to just go in and wipe the Rebels out. The Falcon shows up, his FATHER's ship, and he has more attention put on shooting it down ("they really hate that ship"). Uncle Luke shows up, the man who BETRAYED him, who was supposed to PROTECT him, and the rage becomes blinding.