Please read Part 1 here https://ashwinkumar1989net.wordpress.com/2017/06/28/the-enigma-of-the-half-blood-prince-part-1/, to get a better understanding of my monologue about Severus Snape. So, Snape was a thoroughly complicated character; in my opinion the best character in the series – not according to the moral compass, but in the way he was structured and built up right till the grand surprise at the end. JK Rowling did an amazing job in providing clues here and there (like the Foe Glass reflecting Snape towards the end of Book 4, the Pensieve scene in Book 5 where Lily defends Snape from Potter and Black); at the same time threw a red herring – the conversation between Snape and Bellatrix at the start of Book 6; to try and make us believe that Snape may not be a good guy. Towards the end of Book 6, we got a clue about why Snape had a change of heart after becoming a Death Eater – the way in which the Prophecy was interpreted by Voldemort. By having Dumbledore killed by Snape, Rowling again turned the tables; by making many of us believe that Snape may be a villain. Kudos to her writing(which in my opinion deserves a Nobel Prize) that kept us at the edge of our seats right to the end!
Snape was ultimately on the ‘right’ side in the end, though that doesn’t absolve him from blame for the wrong things he has done. IMHO Petunia was not a likeable character even as a young girl, but I feel Snape was cruel in making that branch fall on her;just because she mocked him. True, he didn’t have good parents(including an abusive father); but I wouldn’t want to physically hurt the sister of a girl whom I like. At Hogwarts, Snape was sorted in Slytherin and gradually fell into bad company of people like Lucius Malfoy, Mulciber, Avery etc. who ultimately turned out to be Death Eaters. However, Lily was the only true friend he had – she eventually made it clear to him that she was not happy with his hanging out with people like Avery,Mulciber etc.; because of their performing acts of Dark Magic. However, Snape countered that by pointing out Potter and Black’s gang’s trouble-making acts; truth be told – which were really bad(they were bullies) but they never indulged in Dark Magic.
However, there was one act of the gang which went too far – Sirius Black leading Snape into a trap in the Shrieking Shack by telling him how to get past the Whomping Willow. Had James Potter not had the sense and courage to himself go in and pull Snape out, he would either have been killed or turned into another werewolf by Remus Lupin in his werewolf form. Of course, it was also true that James was in a way saving his own skin; because- had he not saved Snape, he and his gang would have been expelled from Hogwarts for sure. Nevertheless, credit needs to be given to James for risking his own life in order to save his enemy. Coming to the scene in the Pensieve in Book 5, Lily defended Snape; when he was being bullied and humiliated in the worst way possible by the Marauders – only to be insulted in the worst way possible: “Mudblood”.
Lily didn’t cut Snape off as a friend only because of his calling her Mudblood, she did it because it was the last straw as far as Snape’s hanging out with to-be-Death Eaters like Mulciber and Avery was concerned. She already had to defend Snape to her friends, who couldn’t understand why she even talked with him. We need to understand that Lily as a Muggle-born was in a vulnerable position – being friends with a Slytherin who associated himself with Dark Magic loving and Muggle-hating people. As some fans have pointed out, it is like a black girl being friends with a white guy who is part of a gang of black-hating white supremacists. Also, Snape calling Lily a Mudblood was like a white guy calling his black friend a “nigger”. It is the worst sort of insult imaginable, especially if it is done in public.
So, merely for Snape to apologize profusely to Lily was not enough to obtain her forgiveness. He should have stopped hanging out with his Death Eater buddies – but it was too much to ask of him, as he had an unfortunate infatuation with the Dark Arts right from the beginning. It could also be that, with the combination of being bullied by the Gryffindors and having an abusive Muggle father; he was pushed into the Dark Arts, with Slytherin at that time having a bad reputation for it. Even then, you will rarely get a friend like Lily – a Gryffindor (and that too a Muggle-born Gryffindor) being friends with a Slytherin was even more unthinkable in those days. Snape was in love with Lily – was changing sides too difficult for a girl he loved? He certainly could have taken her advice and stopped being close with the Death Eater gang.
By the time he recognized his mistake(as a Death Eater), it was almost too late. Of course, had James and Lily not changed the Secret-Keeper at the last minute (from Sirius to Wormtail); they still could have been safe. Dumbledore even offered himself as a Secret-Keeper. It is also revealing, by the way; that Snape asked Voldemort to spare Lily; but not James or her son Harry. So Dumbledore was rightly disgusted, though it could also have been that Snape did not want to risk Voldemort’s anger (or suspicion of his loyalty changing) by asking him to spare James and Harry as well. Nevertheless, it was gut-wrenching that Snape wanted to die after Lily’s death, and equally touching that he asked Dumbledore never to reveal his love for Lily; after he agreed to become a spy and protect Harry.
So, Snape’s love for Lily was powerful enough for it not to die along with her death. Was it obsessive? Yes, certainly. Was it creepy (as quite a few fans have said?) I would say no – I feel they are saying it based on what they have seen in the movie “Harry Potter and the Deathly Hallows – Part 2” showing Snape holding a dead Lily. There is no evidence of such a scene in any of the books. Moreover, I felt Snape was raised in such a manner that he did not truly understand love – so he certainly would never have understood the rights and wrongs involved in love. Given all that, it was great that he risked his life so often as a spy, just for love. However, his treatment of Harry in school was certainy wrong – I will be coming to that (as well as Dumbledore’s complicated relationship with Snape) in Part 3 – sorry to stretch it further for you, but I feel it will be less boring that way ;).