You may have read my evil!Filch story: "An Unusually Large Cage." This is another idea I had at about the same time.
Some dialog is, unavoidably, copied from Deathly Hallows, which I do not own. Nor do I own the universe and characters with which I am playing.
Argus Filch surveyed the dead. There they were, arrayed in the Great Hall, all of them killed by Death Eaters. Filch's gruff exterior melted. This was a school - a place of learning! Now, it was a mausoleum.
"So much loss," he mumbled to himself as he wandered off. "So much death. All the time I spent harassing the students. Now some of them are dead - lives cut short by a madman.
"I used to tease them about torture. It was fun seeing their little scared faces, but now... now I don't think I could do that anymore."
Filch was startled by a voice coming out of nowhere: "Do you really mean that, Mr. Filch?" It was Potter under that dratted cloak of his.
Suddenly, Harry Potter pulled off the cloak and revealed himself to the caretaker.
"Potter! Where are you going under that cloak of yours?"
"It's all part of the plan," said Potter. "There's something I've got to do."
"You're going to face him aren't you, Potter?" Mr. Filch said with a surprisingly gentle tone.
"Of course not... this is something else."
"Don't lie to me Potter. I was spotting lying students before you were born. You plan to give yourself over to... him, don't you?"
"Mr. Filch..." Potter began, when he was interrupted.
"Now Potter, I'm sure you think you have a good reason for doing that. Just make sure that it is a good one. I may have given you a hard time as a student, but I have no wish to see you die."
Potter stopped and looked down. "Dumbledore had had backups," he thought, "I should too."
"Mr. Filch. Did you ever wonder why Voldemort came back? His life is tied to his familiar. He's got a huge snake... calls it Nagini. Voldemort can't die unless that snake is killed first. I'm going to meet Voldemort because that's the only way to kill him." It wasn't the complete truth, but none of it was untrue. Harry hoped that Mr. Filch's apparent lie-detecting ability wouldn't sense what he had hidden.
"You don't expect to come back, though?" Mr. Filch asked with concern written on his aged face.
"It's almost certain I won't. Listen. Tell Neville Longbottom, Luna Lovegood, the professors. The snake has to be killed. Ron and Hermione already know, but just in case I fail. In case he comes back here with that snake, it must be killed at all costs. He never lets it leave his side so he'll have it with him."
"I'll tell them, Potter," said Mr. Filch. "For what it's worth, I do hope you come back."
Harry Potter just smiled at him, then slipped on his cloak and vanished.
Argus Filch came across Neville Longbottom and Oliver Wood carrying the body of Colin Creevey. Argus' heart sank. He was just a child.
"Longbottom!" he said, "I need to talk to you. Potter's given me a message for you."
"You know what?" said Oliver Wood. "I can manage him alone, Neville." Wood heaved Creevey over his shoulder in a fireman's lift and carried him away.
"What is it, Mr. Filch?" Longbottom asked.
"Potter's gone to kill the Dark Lord's familiar, a large snake called Nagini. He said that unless the familiar is killed first, then the Dark Lord is unkillable. Said that if he fails, you or Lovegood or someone else needs to kill the snake."
His duty done, Argus Filch retreated to his quarters. There was something he had to do.
In a specially locked cabinet that Filius had given him was a Lee-Enfield rifle. It had seen service in Her Majesty's army at some point, no doubt. Filius had given it to him as a way of "blowing off steam" every once in a while. The diminutive Charms professor had a shorter rifle, of lesser calibre and a more modern design. There was no telling where he had acquired the rifles. On occasion, they would go into the Forbidden Forest to a range Filius had constructed there. Occasionally, Hagrid would join them with an obscenely large rifle that Filius had called an "elephant gun." Fortunately, Filius Flitwick was rather skilled with silencing charms.
Argus Filch mechanically ran through the safety checks on his rifle. Years of practice and an obsessive nature led him to master them easily. Finally satisfied that the Lee-Enfield was in perfect working order, he loaded the internal magazine, slung the rifle over his shoulder, and departed his quarters, locking them behind him.
The caretaker quickly strode to a vantage point on one of the towers. From there, he could see the approach to the castle from the Forbidden Forest where the Death Eaters had made their camp. Argus Filch unslung his rifle, carefully leaned it against the wall and waited.
Some time later, Filch could see a procession of Death Eaters leaving the Forest, their Dark Leader in front of them. He addressed the castle, his voice magically magnified. He was gloating, telling the castle that Harry Potter had died. Filch didn't need to hear that to know it was true. He could see the bundle in Hagrid's arms and knew that Harry Potter had failed in his mission to kill the snake.
Lifting his rifle, Argus Filch gave it a kiss for luck. He brought it into position and peered through the magically-enhanced sniper scope Filius had kindly provided.
Slowly, he drew a bead on the snake's head. He would most likely have only one shot at the snake. He disengaged the safety and put his finger on the trigger. With a well-practiced motion, Argus Filch gently, but firmly squeezed the trigger. Nagini's head exploded.
Rather than doing the sensible thing and raising a shield charm or running for cover, the Dark Lord and his acolytes stood there in shock. There was no spellfire, what had happened?
Argus Filch knew better than to question the opportunity fate had given him. He quickly worked the bolt and sighted the Dark Lord's head. The process repeated and the Dark Lord was no more.
Suddenly, a shout came from the castle as the defenders, happy at their good fortune, charged at the Death Eaters. A furious battle began raging between the Death Eaters and the castle's defenders.
Filch quickly activated the rifle's safety again. It wouldn't do to accidentally shoot an innocent. Enough of them had died today.
Hogwarts' caretaker placed the rifle against the wall, and retrieved a pair of Omnioculars. "What!" he exclaimed. "Potter's alive! He was faking his death. No doubt that was part of his plan." It was all Argus could do to not get up and dance. However, he kept on watching. The defenders were doing well, but he might need to retrieve his rifle again.
The next day, Argus Filch was going about his duties when he heard a throat clearing behind him. He turned around and saw Kingsley Shacklebolt and Harry Potter.
"What can I do for you, Auror Shacklebolt, I mean Minister Shacklebolt?"
The newly-installed Minister for Magic smiled at him. "Our mutual friend Harry Potter, here, has told me that you are the one responsible for the sudden demise of the self-styled Dark Lord."
Filch blushed. "Well, I mean, I suppose I did shoot the snake... and her master."
"That's what I needed to know. Thank you for your service to our community. Of course, you realize that the things Mr. Potter told you are state secrets. We've already spoken to Neville Longbottom. Did you tell anyone else?"
"No, Minister, I haven't," replied Filch.
"Good. I just wanted to make sure that you hadn't. Thank you again for your service to our community, and to the whole world."
Both the Minister and Potter smiled at him and shook his hand vigorously before walking off together.
Argus Filch was still quite fastidious about keeping the castle clean, though he was quite a bit kinder to the students from that point. On occasion, he would even show them his Order of Merlin, if they were polite.
Harry gave more away to Filch than he did in canon to Neville because Neville automatically believed him and Filch was skeptical. He didn't want Filch to think he was lying so he told as much of the truth as he dared.
What of the prophecy and the conventional interpretation of it? Perhaps it was truly fulfilled that night in 1981. Perhaps not.
I thought about having him snipe Bellatrix as well, but I decided two kills was enough.
Thanks again to the fine folks at The Teachers' Lounge, especially Snarky64. We had a nice conversation about indirect objects and that confusing "u" that sometimes, but apparently not always, shows up in British English.