Jerk Pacifist
Frisk smiled. This was going to be a fun night. They had just saved everyone in the underground, and now they were going to kill them all. It had taken a long time- more than twenty resets- but now, everyone was none the wiser to the deed about to be done. Chara was screaming for them not to do it, that they were better than this, they had people who loved them, but they were ignored and shoved to another corner of Frisk's brain. Everyone thought that Frisk was a perfect, angelic child, and Flowey thought that it was Frisk's first time going through the underground. But it wasn't. They knew exactly who they wanted to kill first.
Carefully, they slip out the window, clamber down the tree, and drop into the back yard. There, they slip into the large black raincoat and waterproof pants, and tug on the fine white gloves. No dust will stain their hands this time. Frisk heads into the front yard, and dashes off onto their mission.
Creeping through the night, with eyes used to the dark, they slip into Undyne and Alphys' house. Carefully, they crawl to her bed, and stab her through the head. Dust spews everywhere, but nobody hears, not even Alphys, who is undoubtedly watching some stupid late-night anime. Frisk then glides into Alphys' room, and she falls down, turning to dust in an instant. Just to be thorough, they hack and slash the computer, too.
The next house is the skeleton brothers'. The way the one was always so oblivious, and the other one so hopeless, even when he can't remember the resets… It fills them with hatred. As they wander into the house, the television is on, but nobody is watching. It will muffle their footsteps. Tiptoeing up the stairs, they enter Papyrus's room. Even he has to sleep sometime. One quick slash, and he doesn't even cry out. With careful feet, they unlock Sans' door, and wedge the knife into his lazy skull. Poof, gone in an instant.
As they move on through the other, more minor monsters, their LOVE grows, and they take this burden upon themselves, so the other one can't use the power against them. Hack, slash, side, left. Dust is scattering around, but Frisk doesn't care. This is what they wanted. Not some cute, happy ending, but the way it goes in the real world. As morning nears, Frisk checks their watch. 3:57, still enough time to get Mettaton, Napstablook, and Burgerpants. They won't be awake yet. MTT and Pals isn't too hard to break into, monsters being the naïve, trusting fools they are. The robots both crumble into pieces, and the two dummies are gone in a flash. Burgerpants, sleeping on the "job", goes down easily. As they touch the SAVE point they wear around their neck, it reveals that there is only three monsters left. In a moment of desperation, Chara kicks to the front, and for a moment they are both immobilized for a few seconds. But Frisk's power is too strong, and Chara is shoved to the back, with a final remark of "Four left."
The boss monsters will be hard, but not too hard. What will be harder is forcing Chara to comply with their actions, and making sure that Flowey does not steal the souls. Storing the coat, pants, and gloves in the hollow of a tree trunk that they had found a few weeks ago, they check the time again. 5:23, still enough time.
Yanking on another pair of gloves, they run to Asgore's cottage, and run to the backyard. Panting, they slow to a stop. He's just watering the flowers, and talking to Flowey. Upon Frisk's arrival, he smiles. "Ah, child, why are you up so early?" They bumble out their prepared response. "I was going to visit Papyrus but then his house was unlocked and he wasn't there and neither was Sans and so I went to Mettaton and Pals and they weren't there, and neither was Mettaton or Napstablook and-" Asgore cuts them off. "Are you sure?" "Yes! Absolutely!" "Well then, I'm sure that they will turn up eventually. In the meanwhile, why don't I drive you home?"
Frisk nods, their innocent façade still held up. They slip into the front seat, and just as Asgore starts the car, they slash his body. He collapses and stares at Frisk with an expression of absolute betrayal, and Frisk grins. Chara tries to push them into Asgore's SOUL, but Frisk resists, and the King of all Monsters is dead.
They hop out the car, and dash back to Toriel's house, storing the gloves in the hollow on the way. They tug on their third and final pair of gloves, clamber back into their room, and pretend to be asleep in their bed. She comes into Frisk's room to wake them, and bends over them lovingly.
With one practiced movement, Frisk opens their eyes, and stabs the knife hidden under their pillow into her skull. She collapses and turns to dust, and Chara frantically struggles to make Frisk absorb the SOUL again, but to no avail. It snaps, and now nobody but Flowey is left.
Taking the gloves off, they wipe the dust onto themselves. Scaring him will be so much fun. Heading back to Asgore's house, they shake all of the dust onto their body, step into the backyard, and grin. "It's nice to see you again… Asriel."
A/N: All you guys can decide how this ends. And how humanity reacts. Blah.