It's all you know to do here. It's all you have to offer: your tiny contribution to sanity, to restoring a world in which words like "tidy" and "clean" still make as much sense as "dead" and "corpse".
When the battle began, you were excited--you couldn't help but be caught up in the frenzy of preparation and flight. Your blood, it hadn't pumped quite so fast in years, not since you caught sight--accidentally of course, you weren't no pervert--of them seventh-year boys skinny-dipping in the loch one early autumn evening.
The children--some of 'em--acted like it were a game, smiling and chattering as they rushed about, happy at the prospect of being allowed to brandish them bloody wands at grown witches and wizards, and you felt it, too. Or maybe it were hysteria. Any road, after the shock of hearing that horrible voice in your head--and everyone had heard it right enough, it weren't just you--had worn off, and before the first curse hit the barriers, it seemed to you like you'd stepped right into one of them Muggle pictures Mam sometimes made you go to with her when Dad were away. Which he were a lot when it came clear that there weren't no Hogwarts letter coming for Argus Filch. Funny how you ended up here anyway. If by "funny" you mean "thanks to the old Headmaster", who appeared with a job offer the week after your dad had left for good. Now they were dead, the Headmaster and your dad, both, but you were still here, weren't you? For the moment.
It didn't feel real, and you felt like one of them--whatchacallit?--actors, so you thought you should Do Your Duty, like they did in the pictures. Except in them pictures, the men Did Their Duty by putting on uniforms, picking up guns and marching off to wars far away. This war were coming to you, and the only uniforms were the ones on the students, and you knew that guns weren't much help against wizards, even if you had owned one and knew what to do with it.
As happened so often, it were Mrs Norris what gave you the thought and showed you the way. She wound herself around your ankles and biffed at 'em with her soft paws as if to say, "Remember me, Argus?" As if you could forget your only friend.
So you began trying to round up the castle's cats, to scuttle them to safety, and when you had collected a few, rolling and mewling their way behind you as you dragged the catnip teaser, you asked the Deputy Headmistress where to put 'em, so they'd be safe.
"I don't think it will matter, Mr Filch," she had said, and for once, her voice weren't full of irritation or dismissal, but kindness. "Best to let them go fend for themselves, I think."
And then it were real to you, because she were a right old besom, and no mistake, but she did have a proper care for the moggies, her.
When you watched the grown wizards and witches point their wands to the skies to create the protection, you were in awe. It were so beautiful, like a painting of light. The students, they were in awe too, and for once, you felt kinship with them--all of you standing together in the courtyard, watching the orange beams weave their way over the towers and parapets, bright ribbons of hope against a dark and foreboding sky.
It didn't last. Neither the beauty nor the feeling of belonging.
First the curses began hitting the perimeter, then fear broke the fellowship. The students began pulling their wands, and you remembered again what separated you from them. From all of them.
Then, when the battle began in earnest, when them Death Eaters broke into the castle and brought them unspeakable things with 'em, you were helpless. Even the lowest student could fire a hex to irritate if not disable a foe, and get a chest full of sommat nasty for his trouble, like as not.
The first person you saw die were that little Gryffindor boy.
You were crouching near the central staircase, and the boy suddenly appeared out of nowhere to begin firing hexes at the Death Eater who had been advancing on you, wand drawn. The boy were holding his own against the big--witch or wizard, you couldn't say--when another one stepped out of the shadows. You saw him, but young--Corin? Colin?--had his back to him. You yelled, but it were lost among all the other shouts and booms and cracks of the battle. You even raised what would've been your wand arm--you' d seen the gesture so often by now, it seemed natural--but what could your empty hand do but point and bear witness to the murder of a sixteen-year-old boy who had served detention with you often enough, but whose name you couldn't even remember.
Forgotten by the Death Eater, you stayed crouched in your place behind the rail of the staircase and watched as your castle crumbled along with the bodies of good and evil, young and old, dust shrouding the lot until it weren't clear anymore what were human and what stone.
Then, after the horrible voice came again, and things were quiet for a time, you crept out of your hiding place and went looking for Mrs Norris. You must've been in the dungeons when that Potter boy made his final move and ended the war, because you didn't hear nowt until you emerged, eyes watering, into the Great Hall and saw the survivors, some smiling, some weeping, some bent over the hurt and the dead.
So many dead.
And you wondered, now that the battle were over, what to do. There weren't nowt to do for the dead; you knew that right enough, but the injured? You had nowt even to wipe their brow with, nor a sip of water to give--you couldn't conjure it up out of thin air like the others could. Nor did you have nowt to offer the living, them with already-haunted, empty eyes. You weren't one of them, nor did they like you. You'd made sure of that--contempt being easier to bear than pity--but now you wondered if it were the right course, after all. Mrs Norris were all very well and good, but you craved the touch of a human hand, the caress of a kind word; you knew it now, and suddenly. And any road, Mrs Norris weren't to be found, leastways, not on Hogwarts' grounds. You hoped she got away clean, but you'd probably never know.
You turned away from the scene; it were too painful to keep watching. Not just because of the death and misery, but because you didn't belong, and you'd never belonged. And, you old fool, it weren't because you were a Squib, neither, or at least not all down to that. It were because you wanted it that way, or told yourself you did. The old Headmaster, he didn't just give you a job, did he? He had been giving you a chance to belong. Old Pringle, he had told you his stories about the people in the castle and the times they had, and now that you think on it, most of those stories and times had nowt to do with magic. To be sure, there were some, even in the cloister that were the school, what turned their noses up at a Squib. But not all. Not all. Not until you turned your nose up at them first.
That boy that died in front of you, he knew you were a Squib, but he lost his life protecting you just the same. He counted you among the Good Guys, even if you never had a gentle word for 'im as he scrubbed the trophies or pried that blasted gum from the bottoms of the desks.
As you paced out into the entry hall, you felt a sudden and strong urge to do sommat. To be part of these people what had Done Their Duty and survived it. To Do Your Duty.
And then you spied it: that little pile of rubble near the foot of the central staircase, and you knew what to do.
So you move your broom across the floor of the corridor outside the Great Hall, and if its bristles smear thin, accusatory streaks of blood over the stone, well . . . you'll take a scrub-brush to that, right enough. Wash it all away. Clean. Fresh. New.