Timeline: Third season of AtS, during the episode "Benediction".
Disclaimer: All characters and situations owned by Joss Whedon and assorted ME scribes.
Thanks to: Kathy, for beta-reading; Ruth, for the discussion which inspired it.
There was one sin God could not forgive.
Daniel Holtz had always been a faithful son of the Catholic Church. He also did not suffer from the hubris that plagued Marlowe's Faustus. When he made his pact with the demon Sahjahn, he did not assume this would put him beyond God's mercy because God could not forgive the alliance with hell, but because in order for God to forgive, it would have to be properly repented first. That was the way: to confess, to repent, to do penance, to receive absolution. But if the pact brought him indeed the revenge on Darla and Angelus Sahjahn had promised, Holtz would not be able to regret it, and thus any confession and proclaimed repentance would be hypocrisy. Holtz was many things, but not a hypocrite.
His own damnation, however, would mean that he had to give up any hope of finding his wife and children again in the afterlife. They had died unshriven, but Holtz did not doubt that their time in purgatory had been mercifully brief, and that they were at the side of the saints now, waiting for him to join them. Yet he would never come. He had sacrificed not just his own prospect of salvation, but the eternity with them he might otherwise have had, in order to avenge them.
There was not much still able to make him weep, when he was alone and far from anyone's gaze, but this sometimes did.
Still, though he would have denied it, there was the tiny, flickering hope that he would be able to repent once his revenge was accomplished. That having itself burned out, his hatred would vanish, and that he would be able to beg for God's forgiveness without carrying a lie in his heart. God could forgive all sins, save one, and a pact with hell was not it.
"Suicide," so the priest who had raised him had taught, "is the unforgivable sin."
A sinner could repent anything, could do penance for anything, as long as he was still alive. But not after. To end the life God had given, to die by one's own hand, was to do the one thing for which there could never be absolution.
It took the sight of Stephen with Angelus to make Holtz truly accept damnation.
True, his hand would not be the one to send him to hell, but that was a technicality, and though he liked to call it a last concession to the faith he had been raised in, he knew better. Justine was his right hand in this, as she had been since they met. His was the will, and by ending his life, he extinguished even the last flicker of hope that God might forgive him, and that he would find his wife once more, and little Sarah and Daniel.
Still, he would not be alone in hell.
That was the thing which was different from sacrificing his soul through the pact with Sahjahn. Both times, he knew that his murdered family would be lost to him for the rest of eternity. But they were no longer the only family he had.
As he told Justine, there was love in hell. Holtz had lived through the horror of Quortoth with the child God had given him, the son of the two demons who took everything else. It was only just that this child would remain his through all the fires of all the hell there were after death as well. Stephen would kill the demon who sired him. And he would not repent. Holtz had seen to it. Unrepentant parricides were dammed as surely as suicides ever were.
No, he wouldn't be alone, never again. Damned, but never alone. He would only have to wait a little. And then the child who was his in everything but blood would be his through blood as well. They would be together, and even God himself would not have the power to separate them. How could He?
There was one sin God could never forgive.