Disclaimer: This is fanfiction, therefore it is not mine.
The Memoir of Severus Snape:
Written in its entirety by someone who is most certainly NOT the aforesaid mentioned man.
~Chapter 1: On Death and General Appearances~
The question was never whether or not he could have survived. No, Severus Snape was a brilliant man; he had the credentials to prove it. He'd been potions master and head of the Slytheran house before he was even thirty years old. From there, he would become headmaster of Hogwarts and the right hand man of both Lord Voldemort and Albus Dumbledor. And he didn't get there for nothing, he was brilliant with logic and his intellect was unbeatable. Thus, when the dreadful clammy beast had thrown herself upon him, he had hardly been shocked. Well to put it in his words exactly; "I would have been more shocked if she hadn't"... At least, those would be his words.... if he were not presently dead.
But as always, knowing is only half the battle. Having assured himself that he would in fact be mauled by one giant venomous snake, Severus Snape could easily have prepared a temporary anti venom to assist in his survival of the encounter, whenever it might occur. As well, a simple blood replenishing potion could have been kept on hand, in the case that the anti venom was not strong enough, or he needed to buy some time to formulate a more spontaneous cure for one reason or another. Was not our dearest professor, master of both dark arts and potions, and claimer of the ability to; "Put a stopper in death"?
Perhaps this question is better answered by yet another, 'Why would he want to'?
Yes at any given point in his life and despite his career oriented success, Severus Snape did not have a lot going for him. He was hardly a popular man, nor gifted with social graces. He was certainly unattractive to make things worse; greasy haired and hook nosed. No, Severus Snape did not have much to look forward to... besides of course, a trial.
And oh! What a trial it would have been! Especially since both his former masters were also, at present, dead; one of which by his very own hand! Despite this, it was likely that, should he put the work into it, he'd eventually come out on top. Perhaps he'd even be awarded a shiny new Order of Merlin for all his fine work, or someone might do him the honour of naming a street after him. Oh, and if he was really lucky, he might even have an entire autobiography written for him; peering into every miserable corner of his long and dreary life.
So it was obvious that, to Severus Snape, the small scrap of hardware he might recieve was hardly worth the years of dealing with the Ministry of Magic and all their dunderheaded servents. Yes, perhaps it was better just to die.
So, what if anything can be learned about the end of the most sour and foul Severus Snape? Sometimes, something that seems like and end is merely a beginning, and so with that, sometimes, not everything is quite as it appears.