Kylo is the absolute king of projection. When he screams at Finn for being a traitor, he’s screaming at his own weakness, his inability to banish the light, and maybe even his own failure to punish Finn on Jakku (remember, Kylo looks right at him and just moves on). Finn had the strength to break free of his past and forge an entirely new path. Kylo can’t, and so he hates Finn for achieving the kind of freedom and purpose he can’t seem to find no matter what he does.
It’s one of the reason they’re such good dramatic foils, honestly.
I love how reylo perfectly balances out hero/villain ship dynamics, while maintaining maximum soft and gentle. You get laser sword duels in the snow and the bad guy asking the hero, with intrigue and genuine amicability, why destiny is bringing them together.