I had missed a key scene in my earlier readings of Harry Potter and the Deathly Hallows, coming in the early Chapter entitled The Ghoul in Pyjamas:
'And the more I've read about [Horcruxes]', said Hermione, 'the more horrible they seem, and the less I can believe that he actually made six. It warns in this book how unstable you make the rest of your soul by ripping it, and that's just by making one Horcrux!'
Harry remembered what Dumbledore had said, about Voldemort moving beyond 'usual evil'.
Isn't there any way of putting yourself back together?' Ron asked.
'Yes' said Herminone, with a hollow smile, 'but it would be excruciatingly painful.'
'Why? How do you do it?' asked Harry.
'Remorse', said Herminone. 'You've got to really feel what you've done. There's a footnote. Apparently the pain of it can destroy you. I can't see Voldemort attempting it, somehow, can you?'
Then, from the penultimate Chapter - in the final confrontation:
Harry: 'Before you try to kill me, I'd advise you to think about what you've done... think, and try for some remorse, Riddle...
'What is this?'
Of all the things Harry had said to him, beyond any revelation or taunt, nothing had shocked Voldemort like this. (...)
'It's your one last chance', said Harry, 'it's all you've got left... I've seen what you'll be otherwise... be a man... try... try for some remorse...'
In JK Rowling's covert Christian supposal, 'remorse' = 'repentance'. And, as with The Good Thief, she is saying that Voldemort's soul really could be saved, even at the last moment, if Voldemort was sincerely to recognize what he had done and repent.
But of course he does not; and in the King's Cross Chapter (in a Limbo between life and death) Harry has seen what Voldemort's ripped, unrepentant and un-saved soul has become:
It had the form of a small, naked child, curled on the ground, its skin raw and rough, flayed-looking, and it lay shuddering under a seat where it had been left, unwanted, stuffed out of sight, struggling for breath. (...)
He ought to comfort it, but it repulsed him.
'You cannot help' [, said Dumbledore].