Hellebore Can't Purge You of the Past
Snape was never really Severus. He rarely got close enough to anybody to be used to hearing his given name. This was true before the duel, but was more so after it. Now, the only people that had called him Severus were dead. So Snape, not Severus, stood behind a workbench in the middle of the crumbling walls that were his private potions lab after the duel, preparing a bottle of essence of hellebore.
His skilled hands added half of the liquid to the murky green sludge in his black cauldron. He watched, mesmerized by its contents which turned a calm blue after releasing a sickly yellow vapor. Inspired and desperate, Snape grabbed the bottle and poured what remained over his hands, rubbing them together in a frenzied ablution. There was no sudden calm or abrupt release of anything corrupting and ethereal. He threw the now empty bottle onto the table, smashing his hand into the shattered glass, furious with himself for being hopeful, stupid, human.
The blood ran down his arm in two distinct rivers, separate only until they reached the tip of his elbow from which the red dripped, forming a single, growing pool on the tabletop.
The broken glass didn't seem out of place surrounded by the rubble left after the war. A quick "Evanesco" could have cleared the debris; a few more complicated repairing charms would have been enough to return the room to the way it had been before the final battle. The dungeons—all of the Slytherin domain, had been left in ruins and all of it save for Snape's private lab in the heart of the castle had been quickly fixed by eager, ever happy house elves before the bodies had even been removed.
"Severus, I trust you," Tom had threatened two years ago, intolerant of failure.
"Don't," Snape thought but promised his master the draught of immortality and, by the prophecy's claim, Potter's death.
"Severus, I trust you," Albus had sighed two years ago, weary of his spy's self doubt.
"Don't," Snape thought but promised his master Potter's life and a weakness in the potion.
So, two years ago, Snape had sat in his lab brewing a single potion for both his masters, waiting for the arrival of his apprentice through the floo. His presence in the grate would be enough to trip the school's wards and alert Albus and the Order in the next room of Riddle's imminent coming.
Draco came and unknowingly alerted the Dark Lord that the dungeons were safe through the dark mark on his arm and unintentionally alerted the headmaster that the battle would soon begin.
Kingsley was the first one through the door. Draco, just under eighteen, didn't have to die, and Kingsley, an auror and Order member, wasn't supposed to until at least after the battle had begun. But all false bravado and very real fear, Snape's godson shouted the killing curse. He missed of course, but the power behind it knocked him back into the potions shelves. He fell unconscious, as glass ingredient bottles shattered against his pale flesh—such a very muggle way to die. The leaking potions stock covered the body in a deadly green soup that prevented Snape from coming too near his dying apprentice.
Tom came through a moment later, sealed off the potions lab, used a cutting curse to torture the auror for information and effortlessly cast an Avada Kedavra, killing the man. After checking the potion's progress, Tom left Snape to his brewing and joined the fifty odd Death Eaters attacking the Order in the Great Hall.
Snape remembered adding fresh unicorn blood to the potion while together, Draco and Kingsley's deaths painted the floor an unsettling, Gryffindor red.
The potions master shook his head, pulling himself back to the present. No, he would not clear the rubble from the lab for some time yet. After washing the blood from his hands, Snape walked deliberately to the staff room. He hoped McGonagall wouldn't have anything for him to brew.