Along with the more visible reasons, one possible motivation for the insistence that Kylo Ren is the hero of the saga may be a Christian attitude
toward morality and evil. In areas where Christianity is wielded as a tool of cultural dominance, the Christian teaching of redemption and forgiveness has frequently been twisted into cheap grace–the idea that you can be forgiven for anything if you’re sorry enough, and what’s more, you don’t have to be held accountable or change in any meaningful way.
Another factor may be the
contrast between the Christian and Jewish concepts of evil that @jewishcomeradebot talked
about: Evil tends to be an otherworldly, demonic thing in Christianity but is an
all too human phenomenon in Judaism, and Ren’s character in TFA is an example of the latter (link, current link to full post).
Take these together and, for
large portions of a Christian/Christianized audience, someone who
actually feels sorry or conflicted can’t be truly evil because they are too
human and still redeemable. Since Kylo Ren is obviously human and feels conflict about his actions he is just a “sorry” away from a get out of jail free card and the hero slot. It doesn’t hurt that he’s an able-bodied white cis man, either.
In contrast, Finn in TFA is a terrible Christian hero, at least if we look through the lens of cheap grace and Christianity as cultural dominance. He isn’t shown feeling enough conflict for acts
like killing and lying. He isn’t torn up about his fundamentally
shameful and sinful nature like a good Christian redemptive hero
should be. When he does speak to Rey about the shame he unfairly felt from his
abuse it’s in the past tense, though of course the kind of treatment he has suffered will reverberate for a while yet.
As I discussed in a meta arguing Finn stands for the Balance in the Force (link), Finn does not beat himself up even for his more morally complex acts, either. He fights and kills Stormtroopers
in self-defense and the defense of others, but makes no soliloquies about
how he is a monster destined to kill. He makes things right after
lying to Rey by coming clean to her in a confession that obviously cost
him a great deal, and his conscience is clear. He doesn’t even pretend to be sorry
about misleading the Resistance so he can get to Rey, but he makes up for it by handing them a huge victory. He’s an actually
upright if complicated man who acts on his own moral code, and
he doesn’t feel the need for redemption or salvation.
Finn’s uncompromising dignity, his utter rightness with himself, may be
one of the few unforgivable sins in the kind of Christian framework I described.
Mass murder and genocide can be forgiven if you’re sorry enough, but
failing to suffer from your own sinful nature and not needing a lord and
savior? That’s a bridge too far in some people’s eyes. A
man like that is too free and independent, and cannot be controlled by shame or threats. The
antipathy for this kind of independence can interact in toxic ways with
racism and antiblackness as well, because Finn in Earth terms happens to
be from a group that society says are lesser and should be fundamentally ashamed of themselves.
Of course there is a great deal of racism at play in the fact that large sections of fandom see a mass murderer and patricide as the anointed hero of the galaxy, while at the same time dismissing a conscientious and brave hero as either a violent monster or a minor character of no importance. In addition, however, there may be a cultural divide in that Kylo Ren comes in a more familiar mold to many members of the audience–that of the (white) redemptive hero who can get away with literally anything, for whom grace is so cheap as to be free. Finn in TFA, on the other hand, is something altogether different and, in the eyes of many, worse: Someone who is at peace with himself where he has no business being. Perhaps it is no surprise, though no less sad, that the sequel felt the need to violently punish and mock Finn for the “sin” of his pride.