Severus Snape was not a nice man. Oh yes, Dumbledore forced most of Snape's attitude towards the students with Snape's role as spy, but it would be fooling anyone to believe that at least some of it was natural. But who could blame him really? Mistreated from an early age by his parents, sorted into Slytherin House and bullied by Malfoy, a headmaster who won't punish students who attempted murder (and yet who later forced him to commit murder himself), who could blame Snape?
But yet through it all, Severus never stopped hoping that one day he could have a normal life. Free of the Dark Lord. Free of Dumbledore. Free of those idiotic students he had been forced to teach (as anyone who knew Snape at all could realize – teaching was NOT his preferred career choice). Even his wanting the DADA position was all a ruse set up by Dumbledore to keep Voldemort from suspecting Snape. All Severus really wanted out of life was some peace and quiet and no expectations from other people. And so – despite the fact that he knew Dumbledore had left some evidence that would exhonerate him, he began to plan for an opportunity to fake his own death and escape the wizarding world once and for all.
What potions master in their right mind, whom is often in the company of an insane dark wizard with a pet snake would not keep antivenom on their person? For that matter he had done some experiments early in his potions mastery using snake venom as a potions ingredient, and had built up a slight immunity to it as well. On top of this, the one thing Severus could do – even above his potions skills – was act – how else had he managed to stay spy for so many years? And yet – it was still so easy to pull off. Granted Potter was a bit distracted at the time, given the battle that was raging. And Snape's choice of letting Potter know of his one time love for Lily only helped things along – by the time Potter would be thinking clearly about the whole situation, Severus intended to be long gone – never to be seen by the Wizarding World again.
And so it might have been if it was not for one witch.