Friendly warning again: Spoilers ahead! Read on only if you have read all seven of the Harry Potter books.
ƸӜƷ *...*...*...*...*...*...*...*...*...*...*...*...*...*...*...* ...*...* ƸӜƷ
Finally, the chapters whence-forth Snape was to let himself be loathed by all: The Lightning-Struck Tower and The Flight of the Prince. Whereas up till then, he had to overcome his negativities to perform good, Snape now had to act in spite of his good side. In order to save Dumbledore from a painful and undignified death at the hands of the likes of bloodthirsty werewolf Greyback Fenrir and manic Voldemort-worshiper Bellatrix Lestrange and to protect Draco, Snape was forced, under the directions from Dumbledore himself, to kill with his own hands the headmaster who had guided him to the right path and helped him to fulfill Lily's mission for the past 15 years. It was certainly a task Snape carried out with immense torment - hatred and revulsion etched in the harsh lines on his face, like how Harry hated himself and revolted when forcing Dumbledore to take Voldemort's toxic liquid just a chapter ago. Yet, in spite of his inner suffering, he had to continue his act as a traitor, profound pain throbbing on deep within him, like he were Hagrid's dog trapped in the burning hut.
With his own wand, he had also destroyed the only person who knew of his unwavering true allegiance and condemned himself a betrayer and unfeeling Death Eater, subjecting himself nowon top of a merciless and heartless Dark master on one side danger from his own real allies, including his charge Harry, this time round.
Nonetheless, when Harry attacked him as he was making his escape from Hogwarts, Snape did not forget to dispense his last guiding pointers to Harry: practise Occlumency, master non-verbal spells, stay away from Dark Magic. A teacher he was till the end.
Of note too was that Snape stopped a Death Eater from exacting the Cruciatus Curse on Harry, bellowing as he did. He himself did not fight back either when Harry was hurling spells at him. This extraordinary pursuit scene, so unlike that which deluged dramas and films common in these days, gave food for thought on the true meanings of courage and cowardice. Conventional wisdom has it that fighting equates bravery and scampering, cowardice. Snape has made us look beyond that superficial display of courage and ponder: perhaps the act of not fighting backin order to uphold an ideal and bracing for humiliation instead, is the real manifestation of courage.
In the last book of the series, the Deathly Hallows, Snape became the headmaster of Hogwarts. On pretense of suppressing dissidents, he re-enacted the anti-student group decree issued by the Inspector he detested, in order to protect the students. When Dumbledore's Army schemed to steal the (fake) Sword of Gryffindor, he carted it off to Gringotts to prevent the DA members from risking more punishment. When students got into trouble, he sent them for detentions with Hagrid, even though he knew the latter's allegiance very well, so that they could escape the more cruel alternatives by the Carrows. To deliver the actual Sword of Gryffindor to Harry, he got Phineas Nigellus Black to track his location, and then sent the patronus that represented Lily to Harry to guide him to the swordall without the young wizard's knowledge.
When news of Harry's return to Hogwarts got to Snape, Snape got into action to track Harry down so that he could shield him from Voldemort. On top of that, knowing Harry's affinity for Dumbledore, Snape set the password to the Headmaster's Office to the former headmaster's name so that Harry could seek counsel from Dumbledore easily.
Snape finally had his long-expected fate coming to him, when Voldemort, capricious and holding no regard for his followers' welfare, took his life meaninglessly for a most astoundingly selfish purpose. In his final moments, Snape underwent a painful and protracted death as he struggled vigorously in spite of his wound and draining energy to deliver his last mission to Harry.
That was Professor Severus Snape. Headmaster and protector of a school which collectively misunderstood him. Teacher and guardian to the end of a boy whom he never liked and who himself deeply loathed him. All for his lifelong undying and unconditional pure love of a woman who will never love him in return. And all that he asked for as he departed from this world was to look at her eyes once more.