- Devils are angels who rebelled. They rose up against the deity they served and murdered him. The crime of deicide is unimaginably perverse for angels, and hence devils were cursed and imprisoned in the Nine Hells.
I am wondering how many were the original devils and how long ago did this rebellion happen, since the answers to these questions could make a lot of difference in the cosmology.
- The Nine Hells are what became of the murdered deity's divine realm after his death.
Unless the Nine Hells in 4E are much smaller than now, I'd guess that, for some reason, Baator expanded considerably after the deity's death; the alternative implies that 4E gods are (or at least this god was) much more powerful than the current norm.
I think you're making a mistake in assuming that just one god was betrayed and killed. For all we know, this has repeated itself continuously throughout history, with the newly turned devils joining already existing devils. If I had to guess, I'd say the Nine Hells consist of nine separate godly realms, each of which had an angelic rebellion that led to the death of the god they served.
As for real world, I don't mind. In the Time of Troubles trilogy, we have not only the Faeruinian gods, but an Overgod, Ao. It's then discovered that Ao answers to a god greater than even himself. So one can imagine that there's a Supreme Being at the end of the line, who rules over the whole universe. No reason not to say that that god isn't the God of Abraham, Isaac, and Jacob.
And bear in mind, the article says the following:
The crime of deicide is unimaginably perverse for angels, and hence devils were cursed and imprisoned in the Nine Hells.
- The Nine Hells are what became of the murdered deity's divine realm after his death. The Hells are the devils' prison, and it is difficult for them to get out without mortal aid.
They were cursed, so who cursed them? They were imprisoned, so who imprisoned them? My theory is that it was the Supreme Being (The Over-Over-Overgod). He was the very first deity to be betrayed by his angelic servants, but He was so friggin' tough that they couldn't take him.

So he cursed them into becoming Devils and imprisoned them in hell. Then some other angels worshipping some lesser god (Your average polytheistic deities) betrayed their god and murdered him. So God punished them all just as he did the ones who served him, as their job was to serve that deity, not murder him. Those events repeated themselves with other deities, and God cursed those angels as well, transforming them into devils and imprisoning them all in a hell shaped from the realm they were supposed to defend.
That's my theory, anyway.