There has never been a story with an outright demon as a good guy that wasn't complete garbage
Hazbin Hotel. Disenchantment. Spawn. The Good Place. Your argument is invalid. I mean, there is LITERALLY an entire trope named “Ascended Demon”.
Even slightly less literally, Baldur’s Gate III. During a potential good ending for the Dark Urge, demigod child of the God of Murder; they defy their father and destiny to such a point, that the God of Death himself is genuinely impressed by them and frees them.
But seriously though, I sadly understand why Fundies are against the idea of Demons being good or getting redeemed. Because if Demons, a race born from sin, can reject sin and become virtuous, then there is no excuse at all for humans to claim “it was stronger than myself” or “the devil made me do it”. Fundies cannot act anymore like merely accepting Jesus as their savior and being judgemental assholes still will be good enough to earn salvation.
To quote Paarthurnax(*); “What is better? To be born good, or to overcome your evil nature through great effort?”
(*)who comes from an “always chaotic evil” race himself and whose name literally means “Ambition Overlord Cruelty”.
Besides, if Angels can fall and become evil, why can Demons not ascend and become good? Or are you saying that evil is stronger than good? Isn’t that counter to Jesus’ parables?