I don't have any of my books with me at the moment and I'm writing this in Notepad, so apologies if I get quotes from the books wrong or misremember things - I'm doing this from memory. :)
Unfortunately, to resolve most of the thoughts I've been having we are missing several vital pieces of information, which I hope against hope will be in book 7:
- What Regulus was asked to do that made him back out and leave the Death Eaters
- Similarly, what made Snape decide to leave (it could be the same thing, depending on whether or not Snape knows about the Horcruxes)
- What made Peter join the Death Eaters (was it just fear of Voldemort and/or lust for glory?)
However, we can theorise until we're blue in the face - we might all be completely wrong but at least we'll have a lot of fun in the process. And hey, you never know, we might just get some of it right, in which case we can reserve bragging rights for after the release of book 7. ;)
So, let's have a look at these one by one - personally I've got a few thoughts/possible scenarios which I've outlined below but I'd love to hear some other ideas. I know there are as many possible situations are there are pages of OOTP (that is to say, a lot), so let's play around with them.
What Regulus was asked to do that made him back out and leave the Death Eaters
After what we were told in OOTP about Regulus Black (which was admittedly not very much), it looked very much like he was somehow connected to the Prophecy/Voldemort's downfall/everything happening around that time, as his date of death was described as 'some fifteen years earlier'. It was suggested by many readers that Voldemort might have asked Regulus to betray or even kill Sirius, and that was what made him decide to back out.
However, with the publication ot HBP and the Black Family Tree, dates have been clarified and we now know Regulus died in 1979, just before all the Prophecy-related stuff kicked off. The revelations of HBP (assuming that Regulus is R.A.B.) would seem to suggest that it was finding out about the Horcruxes that made him back out and leave the Death Eaters. We don't know what it was Voldemort might have asked him to do in connection with the Horcruxes, but whatever it was, it must have been that which made even one of the Voldie Youth think it was going too far.
We can be reasonably sure that Voldemort did get Regulus to do something, or possibly several things, connected with the Horcruxes, as it's clear that Regulus knew about the locket and was able to get there and switch the real for the fake - it could have been that he was the one to put the real one there originally on Voldemort's orders and put the fake one there instead from day 1, or it could be that the original was placed there either by Regulus or someone else, and later on he went back to the site/went there for the first time and made the switch. Either way, it's clear Voldemort didn't know Regulus had switched the lockets, since the fake remained in the cave until Harry and Dumbledore discovered it over fifteen years later.
Sirius says in OOTP re. Regulus: "...when he saw what Voldemort was prepared to do to gain power, he tried to back out of what he was being asked to do..." This sounds very much like Regulus finding out about Horcruxes. He (Regulus) might not have known that Voldemort was splitting his soul into seven pieces, more likely he thought the locket was the only Horcrux, but it looks like he still knew about it.
How will we find out this information in book 7? Well, there are many possible sources - Snape would be in an excellent position to give information but it's rather unlikely Harry would sit down with him for a chat about the olden days unless their situation changes drastically in book 7. Lupin was also at school at the same time as Regulus (though as Regulus was in Slytherin and didn't get on well with Sirius it's unlikely they had much to do with each other) and heavily involved with the Order - and as a friend of Sirius's he'd probably also know quite a bit about the situation even if he didn't know Regulus himself all that well (he did say 'Sirius's brother Regulus only lasted a few days as far as I can remember' about Regulus's desertion form the Death Eaters, so it' s possible he knows more) - but the best person, in my opinion, for giving us information about Regulus is Horace Slughorn. He was the one who first told Tom Riddle about the Horcruxes, so he clearly knows about them and how they work, etc. He was Regulus's (and Snape's for that matter) Head of House and Potions master, so would have known him well, and likely other members of the Black family too, as they were all in Slytherin during his time. All in all he'd be an excellent person to go to for information.
What made Snape also decide to leave the Death Eaters?
This is an even more difficult question to tackle, because we don't know anything about Snape's actions at that time, except that he overheard the prophecy and delivered it to Voldemort, who immediately interpreted it in his own way and decided that Harry Potter would be the danger to him, would be the one who could potentially kill him, and Voldemort's main goal has ever been immortality and supreme power.
While even a hardened Death Eater might have qualms with killing babies, I doubt it was this specifically that made Snape rethink his loyalties. There was no love lost between Snape and the Potters and I doubt it would have played on his conscience for very long if he were truly dedicated to the Dark Lord's cause.
To be honest, it could have been many things. Whatever it was is tied in with what Snape told Dumbledore/did for Dumbledore that made him trust him completely. He (Snape) then became a spy for Dumbledore - what could have caused such a turnabout in loyalties?
It could be that, like Regulus, Snape began to disagree with Voldemort's methods, though I can't see any evidence to support that. The idea that was stewing in the back of my head was that, maybe, it was Snape who killed Regulus on Voldemort's orders. Fanon often has it as either Bellatrix or Lucius (the popularity there being the irony that it's Bellatrix who ends up killing both the Black brothers), but I do like the idea of it being Snape.
They were in Slytherin together, of course - Sirius says Snape used to hang around with a bunch of Slytherins who "nearly all turned out to become Death Eaters" - this could well have included Regulus. It's also possible that Regulus was admiring of Snape's knowledge in the Dark Arts, which Sirius admits was very broad from an early age. I'm not saying for a second they were best friends or anything of the sort, but it's possible Regulus was the closest thing to a friend Snape had (or at least, one of the closest).
Regulus, for his part, may have seen Snape as a sort of surrogate big brother figure (and I believe this is a popular belief in fanon) - Sirius didn't agree with their family's opinions, of course, and was rather disgusted with Regulus for seemingly having no backbone and believing all their spiel. Snape, on the other hand, agreed with it and was actively fighting for it (by joining the Death Eaters). It's posisble Regulus joined the Death Eaters inthe first place partly because of Snape, as well as for his parents' approval.
This would be yet another reason for Sirius to dislike Snape - while it looks like Sirius gave up on Regulus in the end, Sirius would have no doubt been angered by Snape seemingly leading Regulus down the path of the Death Eaters - not only was he getting fed those beliefs at home, but now he had Snape constantly reinforcing them at school and influencing his thoughts. (I very much doubt he was the only one doing so, or indeed the most significant one, but it would probably have seemed more irksome to Sirius what with his and James's existing feud with Snape.) Snape might also have felt some small satisfaction knowing that Sirius Black's younger brother held him, Snape, in higher regard than Sirius (not necessarily true, especially when you look at Regulus's eventual fate, but it certainly would have appeared that way).
So, Snape being ordered to kill Regulus for desertion would have had to have been very difficult for Snape - killing Muggleborns and foolish, Muggle-sympathising Wizards and Witches is one thing, but when you start having to murder your own friends you would start to question if what you were doing was the right thing. This would also mirror the situation at the end of HBP nicely, where again Snape has to kill someone he calls a friend, someone he trusts and who trusts him (if you read the situation that way, of course) - which would have made Harry's accusation of cowardice even more rankling, as not only has he had to do it now but he's done it before, too, and it's the hardest thing in the world.
I believe Snape saw a reflection of Regulus's situation in Draco's, too, which is what motivated him to risk his life to help Draco, and make the Unbreakable Vow with Draco's mother (and Regulus's cousin), Narcissa. He was unable to protect Regulus in the end, whether he killed him or not, and it seems like he's determined the same will not happen to Draco (another reason it's so interesting and fitting that Draco and Regulus are actually related). I see many similarities between Regulus and Draco and their situations, but that's another essay.
I feel like this could be yet another reason why Snape hates Sirius, but I can't quite put my finger on why - it could be as simple as a physical resemblance (a la James --> Harry), or it could be Snape feels that Sirius wronged Regulus, though I can't see that being a reason as I'm sure Snape would feel that Regulus was better off without Sirius in his life (whether or not Regulus agreed is another story). It's also possible that Snape feels that the only person who could have saved Regulus is Sirius, and so partly blames him for his death, which is totally unreasonable and not particularly true but that might well have been how he'd seen it at the time and did subsequently.
(... This is probably, as JKR would put it, not a 'profitable line of enquiry', anyway.)
What made Peter Pettigrew join the Death Eaters?
(This is a much harder one to get my head round - I can understand Regulus's and even Snape's motivations better than I can Peter's - despite that I find him a fasicnating character and I hope we find out more about what sent him into Voldemort's service, as I'm hoping it's not as straightforward as it has been presented so far.)
We're never told explicitly why Peter joined up, and there may have been other factors involved such as threats to the safety of his family and/or friends, but it's entirely more likely that it was out of fear, and a desire to protect himself (and maybe his loved ones?). I'm sure they would have kept him on side partly with promises of glory and power when it was all over (power being something he never had within the Marauders, really - camaraderie, friendship, loyalty and even love, certainly, but he never really had any power, and may have jumped at the chance to have some), and partly with threats.
Interestingly, Sirius mentions in POA that Peter had been passing information to Voldemort for a year before James and Lily died, which would have been a few months after Harry was born. Voldemort would have jhust heard the prophecy, and would have started plotting to go after baby Harry. He'd need a spy, someone close to the Potters, to get this information.
However, the Order somehow found out that James and Lily were in danger, and arranged for them to go into hiding and be put under the protection of the Fidelius charm. How did they know this? The Order had spies working for them, certainly, but none who knew about the Prophecy - except Snape. Could it be that Snape told Dumbledore how Voldemort had interpreted the prophecy? Dumbledore had heard it from Trelawney, yes, but he wasn't to know that Voldemort would decide upon Harry Potter as his target, not until Voldemort actually struck, anyway. Dumbledore said that he was a sufficiently skilled Legilimens to know when he was being lied to, so he would have known Snape was telling the truth - could this be the act that caused Dumbledore to trust Snape so implicitly? Possibly not, as Dumbledore told Harry that Snape still felt he owed James his life since the Shack incident, which he wouldn't feel if he had saved them by telling Dumbledore they were in danger (even though it was he who put them in that danger by repeating the prophecy to Voldemort).
If Snape was indeed working for the Order and passing them legitimate information, he obviously didn't know that Peter was the spy, as Peter was not suspected by anyone on the Order's side - we know both Remus and Sirius suspected each other, for example - which is why Peter seemed such a good choice for Secret Keeper. So yes, Voldemort likely would have started pursuing James and Lily's friends to get information. No point going after Sirius, it was pretty widely known that he would die before divulge any information, and besides he was suspected already, as was Remus. Peter seemed like a good choice, plus he was easily swayed and not as close to the Potters as Sirius (i.e. wouldn't be noticed as much). Who knows if some of their other friends, particularly Lily's conspicuously absent friends, were approached and refused? They'd have been killed, of course, which is why we wouldn't have heard of them (or they could be some of the deceased members of the original Order).
(I was also wondering if there was any link between Peter and Regulus, but the timescales don't fit - Regulus died in 1979 and, if Sirius's verison of events is accurate, Peter started passing information to Voldemort in the autumn of 1980.)
Peter's bound to make an appearance in book 7 - there's still that pesky life debt hanging over his head - and I for one am hoping that when he dies (for surely he must) it will be to save Harry and/or Remus, and he might finally live up somewhat to being put in Gryffindor.
I also hope that Draco manages to avoid Regulus's fate, though whether he will actively fight against Voldemort as Regulus ended up doing remains to be seen.
So yes, I realise this is all a bit disorganised, but - any thoughts?