A lot of the specifics of what went down with in Galactic Civil War aren't exactly public record. Even Kylo doesn't know a lot of the details for certain. So he feels pretty comfortable, ah, 'speculating'.
(Fantasising)
He knows that his parents had a rocky relationship, and that his existence was part of that. He knows his power always made his father uncomfortable. He knows he was conceived around the time of the Battle of Yavin. He knows his mother was held captive and tortured by Vader on /a/ Death Star (he doesn't care to think about which is the likely one). He knows Leia looks a lot like her own mother, and that Leia and Vader had no idea they were related.
And that everyone always said he had too much of Vader in him.
tl;dr Kylo likes imagine that his mother was raped by Vader when she was interrogated, that she got pregnant with him as a result, and this was the cause of all the strife in his early life. It's also probably a sexual fantasy.
(would also accept an AU where all this was actually the case, tbh)
[rape & incest w] Kylo + fantasy Vader/Leia - Kylo likes to imagine that Vader is his real dad
(Fantasising)
He knows that his parents had a rocky relationship, and that his existence was part of that. He knows his power always made his father uncomfortable. He knows he was conceived around the time of the Battle of Yavin. He knows his mother was held captive and tortured by Vader on /a/ Death Star (he doesn't care to think about which is the likely one). He knows Leia looks a lot like her own mother, and that Leia and Vader had no idea they were related.
And that everyone always said he had too much of Vader in him.
tl;dr Kylo likes imagine that his mother was raped by Vader when she was interrogated, that she got pregnant with him as a result, and this was the cause of all the strife in his early life. It's also probably a sexual fantasy.
(would also accept an AU where all this was actually the case, tbh)