(This fanfic is written completely in 3rd person to avoid as much confusion as possible.)
Frisk POV
Frisk couldn't help it. They snapped. They couldn't find a way to not die. The monsters in the ruins killed them, Toriel killed them, even though she was the closest thing to a mother they had, Papyrus kept beating them up and throwing them into a shed with dog food. Undyne was merciless, Mettaton killed them on live TV for everyone to enjoy and Sans, with all his talk about promises, didn't do anything about this.
After overcoming all of that, Frisk was still faced with the monster king himself: Asgore. When he fought, it was clear that they couldn't show mercy toward him. So they fought. And died. And kept dying. They just couldn't stop dying. A little voice in their head told them to show justice, to kill them all. Frisk tried to ignore the voice throughout their journey, but now, they listened to it. They had enough of showing mercy to the people who've killed them. They could just come back after all.
RESET
Frisk killed everyone this time. They even killed Sans, after dying too many times to count. They moved forward, to the king. They were covered in the dust of the people who they once showed mercy to. As the walked past where Sans normally stood, they finally understood what they just did over the course of the past day. They collapsed on the spot, sobbing uncontrollably as the full weight of what they did fell on them. Without meaning to, they reset.
Undyne POV
Undyne shot up like a rocket from her bed. She remembered fighting that kid, the one that killed them all. She remembered dying. Why was she here? Did the kid give up? Unbidden, she suddenly remembered how many times the kid actually went back in time. It was so high, she didn't bother to count it. She remembered herself killing the kid a lot too, when they were on the ground, crying, begging for mercy. She felt disgusted at her actions now. She had to find the kid, and fast.
She ran as quickly as she could to Snowdin. The residents there were going about business as normal, so she knew the human alarm hadn't been raised. She continued running through Papyrus' puzzles, and suddenly, she ended up back in Snowdin again. 'Someone killed them.' she realized, and ran even faster, only to be back in Snowdin again. She then saw Papyrus walking down the main street.
"Hey Paps!"
"HELLO UNDYNE! WHAT BRINGS YOU TO SNOWDIN?"
"Where's Sans?"
"IT WOULD SEEM THAT I DO NOT KNOW WHERE HE IS EITHER. I THOUGHT THAT HE WOULD BE SLACKING OFF AT HIS SENTRY STATION OR TAKING A BREAK AT GRILLBY'S, BUT IT WOULD SEEM THAT HE IS AT NEITHER PLACE! I HOP-" the sudden pull in time interrupted him, as she ended up in the spot she was at 5 minutes ago again. This time, she made a dash for it, almost making it to Sans' sentry station before she was pulled back again once again. 'Argh, this is ANNOYING' she thought, as she dashed toward where the kid was again. This time, she made it, and Sans was just standing at the door, his left eye glowing blue.
,
"I hope that dirty brother killer finally gave up." Sans growled
"Who? The kid?" Sans turned around in surprise
"What kid?" Sans asked slipping back into his normal tone
"Don't act like you don't know. You just said they killed Paps, and they haven't this time, so you do know."
Sans grumbled to himself "I thought you didn't know."
"I know now, and I know you've been killing them." Undyne said, her voice slipping lower.
"Why do you care? They killed all of us."
"Because of how many times we killed them." Undyne said simply
"But…" he grumbled under his breath, and the argument kept going
Frisk POV
They crept out of the ruins door, fearful of facing Sans again. He'd killed them 4 times now; he knew what happened before. This time however (and for the first time, they were glad about it), Undyne was here. She was arguing with Sans about something, and spotted them as soon as they tried to go back. She dashed toward them, grabbed them and let out a long speech about honor and how their soul was needed to break the barrier. They numbly wondered if she was going to kill them, until she suddenly ran as fast as possible away from Sans, and in no time at all, they reached Undyne's house.
"Okay, squirt," Undyne said "What's going on?"
Frisk looked at them dumbly.
"Okay, look kid, why is time going backwards every 5 seconds? Who's killing you?"
At this sentence, Frisk looked up at them with tears in their eyes "You know…?"
Undyne nodded "I do now at least. Damnit, I wish I'd remembered the first time I'd killed you. I shouldn't have forced you into that fi-ooph!" Undyne was interrupted as Frisk hugged them tight and cried.
"I'm sorry I-I'm sorry I'm s-s-sorry. I *hic* I'll give my soul to the king, please don't kill me, please, not again, just one last time with the king, please *hic* please please…"
Undyne stared at them in confusion "Squirt, I'm not going to kill you. I'm surprised you kept going as long as you did that first time without killing any of us. I should be the one apologizing."
Frisk stared at her again, before breaking into tears again and sobbing into Undyne's shoulder "Th- *hic* Thank you…"
"No problem squirt. Just remember that if not anyone else, I'm here now. I'm here now."
(I thought about playing with the idea about Undyne remembering, since she's the most determined monster in the Underground, especially in the genocide run.)