Ok,
a lot of us know that jk rowling said there will be an important discovery about lilly potter in number 7.

what if voldemort wanted lilly as a Death Eater? back when snape and james were kids he caleld lilly a mudblood when she saved him. lilly was supposedly excellent at potions too, if snape saw HER as a rival then it wasnt james's fault snape hates harry, its lilly's. When snape became a death-eater and the prophecy was discovered i suppose snape informed voldemort of her capabilities. and snape supposedly was picked on and felt as if he was welcome as a DE, and he was thought of as a right hand man to voldemort.
if he thought of lilly as a competetor. remember what voldemort said to lilly in gordrics hollow "step aside silly girl" why the hell would voldemort say that? voldemort wouldnt be concerned of anybodys life unless he needs to use them.
Snape probably witnessed the murders at Godric's Hollow. After seeing what had happened with the failed AK curse, Snape, with his knowledge of Dark Arts and LV, knew the DL would not stop until he had found a way to inhabit a body again. The callous disregard in which LV had dismissed the Potters' lives and potential usefulness to him, may have made Severus realize his own position. Snape's Dark Arts virtuosity, commitment, and role of close personal advisor and spy, meant nothing to LV the sociopath. Snape could be just as summarily dispatched with as the Potters, at LV's whim. Snape may also have realized the DL's tactical weaknesses, created through his arrogance and perverted use of Horcruxes. Should LV be able to regain a body, he could fail in his final attempts through his errors, and then Snape would fall into the hands of Albus Dumbledore and the Order. Snape had to bide his time and act judiciously until he saw what would happen with LV regaining power. Snape would act to save his own skin, in Slytherin-like tradition.

Snape’s only hope? Dumbledore. Dumbledore, after hearing him out, realized that Severus would aid and abet the Order of the Phoenix only because it would aid his own survival, not because it was the right thing to do. Even if LV came back into power and defeated Dumbledore and the Order, Snape still might not survive if he was on the DE side. LV would countenance no rivals, and Snape might know too much for the DL to tolerate him past a certain point. Despite how distasteful it must've been to aid and support Harry and certain members of the Order with whom Snape had an unpleasant history, he knew what he had to do to stay out of Azkaban. Snape joined the Order.

Severus had come to the right conclusion, in spite of himself.

While waiting to see when LV might regain a body, Snape began to develop his double-agent role. He could then take advantage of whichever side might benefit him at certain times, in the future. Feeling as he did toward Harry, it didn't take much acting skill to present a cruel, bullying demeanor to Harry, his friends, and their sympathizers. Teaching in front of a classroom which included the children of DEs must've meant they would tell their DE parents about Snape's favoring of them and all Purebloods. At this time, Snape could be gathering more information and magical knowledge at Hogwarts, since the School had more "...mysteries to unravel, stores of magic to tap." (p.431, HBP.) Concurrently, he had to collaborate with Dumbledore on LV, the Dark Arts, and plot strategy for the time when the DL reappeared. Dumbledore must have felt that this was not a question of if, but when. Snape needed to aid the Order so that Harry's chances of succeeding were bolstered. If the Order triumphed in the end, Snape's chances of surviving were better, too. Snape continued to bide his time, gather information for his own purposes, and keep himself out of Azkaban.
The truth is, i think snape saw dumbledore at a week moment and killed him. he is not loyal to the dark lord, he would have done the same to voldemort. he is neutral on both sides.
and the rab thing, could mean a different thing and maybe snape took it.
maybe it was Regulus Arcturas Black