The believers in the Light couldn't abandon their position because it would force them to admit that all the deaths had been for nothing. That they'd asked people to die for an ideal that ultimately failed.
Rather than face such bitter truth, they became more and more convinced that the sacrifices and losses were somehow becoming so tremendous that they had to become worth it. That the balance of the scales between good and evil would soon tip to favour them, because — it simply must.
It made Hermione leave Order meetings ready to cry with frustration. She even resorted to writing up a presentation explaining sunk cost fallacy, irrational escalation of commitment, and self-justification theory. When she tried to explain muggle psychology it was brushed aside, and when she tried to push it she was treated like she was some kind of craven monster; trying to use psychology to legitimise murder.
She once spent thirteen hours in the infirmary painstakingly reconstructing Professor Flitwick's lungs. When she was called to an Order meeting immediately afterward she went in exhausted, and broached the topic of dark magic out of renewed fury. She'd been angrily informed by an equally angry and exhausted Ron that she was being a bitch and didn't even seem to understand the point of the Order.
Several other members nodded. Harry hadn't, but he refused to look at her, and he'd patted Ron on the shoulder as he left the meeting.
She cried afterwards.
Severus had found her in a storage closet, having an emotional breakdown. After alternating between mildly insulting her and grossly insulting the rest of the Order for several minutes, he'd managed to make her regain her composure.
Flattery by way of restraint.
The next time he attended an Order meeting he had given her a book on occlumency. He hadn't had the time to train her, but Hermione hadn't needed training. Just reading the concepts enabled her to internalise the technique.
Severus later told her he'd suspected as much. She was a natural occlumens. It was part of why she was talented in healing and potions. She had the ability to fully compartmentalise when she needed to.
After five years of war, Hermione felt as though her entire life had gradually become sequestered into various little boxes. Her eternally strained relationship with Ron and Harry was carefully buried in a corner where she couldn't feel it. Most of her relationships felt put away. In the center of herself, in the enormous space her friendship with Harry and Ron had long filled, there was now a cavern that she kept dutifully occupied with work.
After a few minutes, she reopened her eyes and resumed reading. She only had two days left to prepare.
Minerva McGonagall unexpectedly arrived at Grimmauld Place the next afternoon, as Hermione's hospital shift ended. The former headmistress of Hogwarts rarely left Scotland. After Hogwarts had been shuttered, McGonagall had undertaken guardianship of all the underage witches and wizards who were orphaned or whose parents were fighting in the war. She'd returned to her father's manse in Caithness and after abusing expansion charms to an absurd degree, making it large enough to house over a hundred children.
She regarded anyone without parents as being her charge. With Hermione's parents obliviated and hidden in Australia, that meant Minerva regarded Hermione as being under that umbrella as well.
They went to tea in muggle London.
When they had seated themselves, she stared silently at Hermione for a long time.
"I had hoped you would refuse," Minerva said at length.
"Did you really think I would?" Hermione asked, her voice steady as she finished pouring the tea.
"No," Minerva said stiffly. "My hopes and beliefs have been separate things for some time now. Which is why I said it was unconscionable."
"The Order needs this."
There was a silence as each woman studied the other. The tension between them vibrated; like the sob of a violin bow drawn carelessly across the strings. Sharp. Aching. Deeply felt.
After a minute, Minerva spoke again.
"You...were one of the most remarkable students I had the privilege to teach. Your relentlessness back in Hogwarts was always something that I admired—"
Minerva paused.
"But—?" Hermione pressed, preparing herself for the sharp critique that waited on the far side of the compliment.
"But—" Minerva put her teacup back in its saucer with a sharp click, "the way you have carried that tendency into the war has troubled me. I sometimes wonder where the line is for you. If you even have one."
Once — such a rebuke would have made a Hermione blush and reconsider herself. Now she didn't even blink.
"Desperate times call for desperate measures," she said. "For extreme diseases, extreme methods of cure, as to restriction, are most suitable."
Minerva's expression hardened, her lips thinning.
"And what of 'first do no harm'? Or do you think the oath does not apply when the harm is to yourself?"