Author's note: This idea has been in my head ever since I saw the final film. It's taken awhile for me to get right, but I hope you enjoy this final outcome, as well as the subsequent chapters that focus on other characters.
Disclaimer: I don't own Harry Potter

The night she drew her wand against Severus Snape was the first night in a year that Minerva McGonagall had felt alive. It was that year that had dragged on without end; the longest year she had lived through yet, time having indelibly slowed for her ever since the night the man who was her best friend, her mentor, her hero, and the love of her life, had been killed. Ever since Albus Dumbledore was killed.

And his killer stood there before her. Snape had taken his position, his school, in the name of the Dark Lord and showed not an ounce of remorse for it, and, Minerva was sure, he was one step from taking even more: the life of the boy in whom Albus put all his faith and his hope. She wouldn't allow it.

When the man who falsely called himself Headmaster had moved to assault Harry Potter, she refused to stand aside.

She raised her wand and brought forth fire, though the flames she cast were not truly her own. They were the flames of the phoenix, who had sang it's lament upon Dumbledore's death; who had and would continue to rise from the ashes upon it's demise. In this fire she imbued her refusal to let Hogwarts, the school that had become her own and so many others' home, that held so many memories, that Albus had given his life to keep safe, become a mere ruin. However, above all else, she imbued in the fire her conviction that Albus Dumbledore would not – no, could not – ever truly be removed from his school.

She could see the subtle changes in Severus's cold demeanor when she fought back. She could see the fear in his eyes, his surprise that after so long she would stand against him, and his horror when the realization of just what her flames symbolized had dawned on him.

Through this horror, she also observed, for just a moment, a fragment of guilt in his dark eyes; an unrequited, impossible desire to explain himself and his actions. His ensuing felling of the Carrows and refusal to attack her, she knew, was his desperate attempt to make her realize this.

No. The time for sympathy and forgiveness had long since passed. Severus Snape had taken so much from her, she would not allow him to take as well the last hope of both Dumbledore and the wizarding world itself.

Despite his message to her, she continued to cast her spells until the very moment he retreated.

She had no other choice.

For it for Harry, for every student at Hogwarts, for the Order of the Phoenix, for those who had already given their lives standing against Lord Voldemort, and for Albus that she raised her wand that night.

It was for them that she kept fighting.

It was for them that she kept living.