Disclaimer: All these characters belong to J.K. Rowling and her publishers, as does the plot of "Harry Potter and the Deathly Hallows."
This tiny story is written from Minerva McGonagall's point of view. The way I imagine her interaction with Snape has been influenced by the many excellent stories out there regarding his year as headmaster, so thanks to all of you!
The last thing she said to him was, 'Crucio.'
There was a year of bitter betrayal behind it. A year of watching him destroy the school, torture the children. A year of sitting beside him at High Table, knowing he'd killed Albus and that he planned the same for Harry.
"I trusted you," she'd told him in disgust on the first night of the school year. He'd been sitting behind Albus' desk, cool as stone, making notes on last-minute lesson plans – probably the Carrows' travesties.
"A common mistake," he murmured dismissively. He spared her a glance utterly void of the anger that had always defined him. He'd radiated fury since he was a child, and this stranger didn't even have his eyes.
The months passed, and there'd been no smirks, no wit, no impatience, no anger, and no mercy. She'd begged him to call off his thugs, to spare the half-blood children who came to her bloody and nauseous. He did nothing. Yet the castle itself seemed to reach out in protection, in spite of its master.
At times, she felt she hated him more than Voldemort.
Then Harry came, and the reckless release of battle, and she was funneling months of helpless despair into spells more violent than she could honestly excuse. She recognized this, standing over Amycus' bound body and feeling the helpless flutter of his will crushed under her own savage power. She drew back with belated apprehension; she must control herself, for the children's sake. For her own sake, and for Albus' memory.
She fought with charms and hexes, with fire and furniture, with self-control, even against Severus. But when, after hours of struggle, she saw his black-robed figure sweep back through the ruined doors of the Great Hall, as though nothing they'd suffered had made any difference, something dark rebelled inside her. She charged, cornering him before he could even think about approaching one of the children. Her madness gave her strength and speed and unpredictability, but still she couldn't touch him. He was in control, and she was not, and that knowledge fueled her frenzy; she was making beginners' mistakes. After five minutes of fighting badly, she crossed the final hurdle. "Crucio!" she screamed.
He ducked under the red bolt of light, and when he straightened he looked alive for the first time all year. He stared at her, his mouth twisting down in long-absent scorn, and said, "Minerva. How disappointing."
Her eyes blurred with dangerous tears, and he vanished around the staircase in a flash, in pursuit of his own priorities.
Late that evening they brought his body back, mauled and bloody. She was tempted to whisper apologies and regrets, gratitude and compassion.
She settled for closing his eyes.