You know, we were told in RC grade school that even Hitler could get to heaven, if he received the sacrament of penance on his death bed and was absolved of his sins. Then he'd be free to go heaven. But if there was no priest around, he would have to make a perfect act of contrition which isn't easy to do. He'd have to get himself to a mental pitch where the emotions of sorrow and guilt for what he did was at that moment greater than his hate and evil emotions. That's not easy to do. But with training or practice, it is possible.
Considering more than a few murdering assholes became saints ( Olga of Kiev for example), and others were assholes in the name of faith even during their monastic lives (supporting crusades, inquisition and such) I don’t see why would the Church be deterred by a little genocide. Besides that is the basic business model. Invent a metaphysical system where you both create a non existing problem (sin) and than offer the solution to that problem (salvation from the same through your exclusive ritual). Genius actually.
If "sin" is another name for immorality, I'd say it's a pervasive existing problem--Hitler's being an extreme example. I have a consequentialist view of morality in which the murder of six million plus people is monstrously immoral, and a person's death bed change of heart isn't going to erase that! The nun's deontological position emphasizing mental states, is at best a mitigating circumstance. Noting can erase the horrendous damage. But of course, she was just repeating her church's teachings. It's good, I guess, to encourage wrongdoers to repent and change their ways. I just wish he'd done it a few decades sooner. On the other hand, we have the example of the wolf of Gubbio, who terrorized and devoured many residents before being converted by St. Augustine. The wolf repented and mended his wicked ways (or so they say), thenceforth survived by going door to door begging for food from the townspeople, and was given a Christian burial at the Church of St. Francis of the Peace in that town. The wolf's remains were exhumed at the church in 1872. Francis of Assisi Whether fact or fiction (I suspect the latter), I find the wolf's conduct exemplary compared to Hitler's.
I think the Gospel of Peter was rejected for inclusion in Bible canon, some think because of its proposition that eventually every soul will be saved. Such a teaching would seem to fly in the face of Revelations. And it's probably unwelcome to many of us who have little idea of the consciousness of God. But none of us, as far as I can tell, know anything for certain about the Mind of The Spirit (aka God).
The main reason the Gospel of Peter was rejected was because Bishop Serapion of Antioch and others thought it had a Docetic view of Jesus as spirit instead of flesh--i.e., the notion that Jesus' human body wasn't real but either an illusion or made of some supernatural substance. In Favor of Demythologizing the Ethics of Jesus | The Bart Ehrman Blog the gospel, Jesus doesn't seem to suffer on the cross, his soul ascends directly to Heaven at his death, and later a giant Jesus emerges from the tomb, accompanied by two giant angels and a talking cross. "(38) Then those soldiers seeing it woke up the centurion and the elders, because they were there too, keeping guard. (39) And while they were explaining to them what they saw, again they saw three men coming out of the tomb, with the two supporting the one, and a cross following them. (40) And the heads of the two reached as far as heaven, but that of the one being led by them reached beyond the heavens. (41) And they heard a voice from the heavens, saying, "Have you proclaimed to those who sleep?" (42) And a response was heard from the cross: "Yes!" There was no mention that anybody saw Jesus afterwards. Gospel of Peter — Gospels.net The gospel was later denounced as heretical by Saint Jerome and Pope Gelasius I. Modern scholars point to numerous factual errors and anti-Semitic passages, but those don't seem to have been the Church's original objections. Why Shouldn’t We Trust the Non-Canonical Gospels Attributed to Peter? | Cold Case Christianity