Snapedom

In fact, it's pretty clear in HBP and DH that the Malfoys, who appear to have been among his biggest supporters in the past, are obeying Voldemort not out of loyalty, but because they're afraid that he will kill them if they try to leave. Even when they're already under the threat of death in HBP and you'd think that they wouldn't have much to lose, they still prefer the slim chance of Draco completing the mission to the certain death they're sure they'll face if they defect to Dumbledore's side. (To be fair, Narcissa is also counting on Snape and the Unbreakable Vow to help protect them.)

So why would the Death Eaters have agreed to serve such a leader in the first place? I have to imagine that Voldemort wasn't always like that, and that he must have been capable of using charm and persuasion in the old days. It seems likely to me that he was losing more and more of his humanity with each successive Horcrux, and by GOF, even his body is no longer truly human--it's a magical construct. It doesn't seem surprising to me that along with his body and soul, he's also lost his ability to feign the kind of human emotions that he initially used to recruit his followers.

I also think that Snape's initial recruitment was probably done by Lucius Malfoy and maybe some of the Slytherin friends that Lily disapproved of, like Mulciber. I suspect that new recruits didn't get much face time with the Dark Lord until they gained more experience and proved themselves. However, he must have proved himself eventually, because his arm bears the Dark Mark, and we are told in DH that not all the Death Eaters have it, only the Dark Lord's inner circle.

I'm not quite sure this means that Snape was Voldemort's right hand man at the time--he was still very young and relatively inexperienced, after all. I suspect that Voldemort saw him as a promising young recruit, and possibly--perhaps unconsciously--identified with Snape's half-blood status. Voldemort did, after all, assume that baby Harry was the greater threat, even though Harry has Muggle (or at least Muggleborn) blood, and Neville was a pureblood. So maybe he had marked Snape as someone to watch, someone that he expected great things from--a not unjustified expectation, since Snape was already a Potions genius and creating his own spells while still a student.

Whether or not Snape was actually Voldemort's right hand man during the years before Harry's birth, there is evidence that Voldemort valued him--as mentioned above, Snape was given the Dark Mark. And also, Voldemort agreed to spare Lily for Snape's sake--and more importantly, actually attempted to keep his promise. That more than anything convinces me that Voldemort thought Snape was valuable enough to humor his infatuation with a Mudblood girl, or perhaps he thought Lily might be useful as a hostage to control Snape and ensure his loyalty.

However, Voldemort's human veneer may have been wearing thin at this point, since he had completed most of his Horcruxes. Whatever the reason, he must have shown that he wasn't trustworthy enough for Snape to leave Lily's safety entirely in his hands, prompting Snape's defection to Dumbledore.

After Voldemort's return post-GoF, I think that Snape eventually did become his right hand man, or at least, the pawn that he favored the most, since I don't think that Voldemort really valued his Death Eaters as anything more than useful pawns at best. The fact that Bellatrix seems jealous of Snape indicates that Voldemort has been showing Snape more favor than he does even to his most loyal and devoted follower. And Voldemort seems to show a touch of regret when he kills Snape in DH--in a very shallow, offhand way, but it does seem to be genuine, although it's more along the lines of "it's too bad that I have to destroy this useful tool" rather than feeling sad about having to kill a comrade or even a favorite servant. He also still believes that Snape was loyal at the time, and doesn't seem to consider the possibility that Snape was double-crossing him until Harry defiantly tells him of Snape's true loyalties during their final confrontation.

So how did Snape rise to the rank of Voldemort's most trusted servant? When Voldemort gathered his Death Eaters in GoF, he referred to one of the absent Death Eaters as "One who I believe has left me forever...he will be killed, of course". Assuming that he was referring to Snape, how did Snape go from traitor to right hand man?

Partly with some smooth talking, I believe--he tells Bellatrix in HBP that he managed to convince Voldemort that he delayed answering Voldemort's call in order to ensure Dumbledore's trust and strengthen his position as a spy. He claims that Voldemort was convinced; I think that Voldemort was convinced enough not to kill him, but probably kept a close eye on him after that and didn't immediately trust him. Having a spy that close to Dumbledore was probably worth enough to Voldemort that he would let Snape live long enough to prove his loyalty.

And it's pure speculation on my part, but I've always thought that Voldemort was impressed by the fact that Snape had the courage to return to him after the delay, knowing that he faced the possibility of torture and death if Voldemort didn't buy his excuse. I mean, you don't see anyone calling Wormtail "the Dark Lord's right hand man," even though he was instrumental in helping the Dark Lord create his new body. Voldemort rewarded him with the silver hand, but seems to regard him with contempt. (And the hand turns out to be not such a great reward after all in DH.) Of course, Snape would have to walk a pretty fine line--courageous enough to win Voldemort's respect, but deferential enough that his courage won't be seen as a challenge to Voldemort's authority. However, I think Snape is skilled enough to pull that off.

If we take Snape's explanation to Bellatrix at face value, he was able to provide Voldemort with valuable information--or at least he was able to convince Voldemort that his information was valuable. It's not clear whether he actually offered up some of the Order members as necessary sacrifices in order to seal his place in the Death Eaters, or whether he was able to somehow take credit for their deaths even though he didn't intentionally cause them. (I'd say the latter, since he seems worried about tearing his soul by killing Dumbledore. I don't think he'd be as worried about the state of his soul if he'd already committed murder, even indirectly. On the other hand, he does consider himself responsible for Lily's death...but I guess that's a topic for another essay.)

I think the fact that most of the Death Eaters were captured and imprisoned at the end of OotP also helped Snape's rise in the ranks. For one thing, the number of his rivals was immediately decreased. And even after they were freed, they would bear the stigma of having failed in their mission by (1) not getting the Prophecy as they were ordered, and (2) getting captured. We see in HBP and DH that Lucius has lost his position of power in the Death Eaters, and Voldemort punishes him for his failure by giving Draco a near-impossible task that will result in the entire family's deaths if he fails.

In contrast, Snape managed to remain free (if only by virtue of not having participated in the raid on the Ministry) and continue to make himself useful. It also seems like he may have shared spells with Voldemort--in DH, we see them both flying without a broomstick. If Snape has the ability to create such powerful spells, he would be valuable to Voldemort, and if Voldemort is the one who created the spell, it shows that he must favor Snape highly, since he seems to be the only one that Voldemort taught it to. (I include the possibility that Snape created the spell because he seems to have been creating his own spells, such as Sectumsempra, during his student years at Hogwarts, so he may have continued with his research as an adult.)

The power and position that Snape might have craved as a newly-recruited Death Eater probably seemed bitterly ironic to the older and more cynical Snape. It would mean little to him now, without his beloved Lily. And the people whose side he was truly on scorned him as a traitor and regarded him with contempt. Of course it was all part of the plan to have everyone on the Order's side believe that Snape had betrayed them, but at the same time, it must have hurt to have everyone believe it so easily.