Self-Defense -AU-

Frisk trembled before Sans as the child stood over the dusty remains of Asgore.
"kid...why did you do that?"
The sick memory of seeing those coffins, one empty with their name on it...
"you were so close to that final ending! the good one!" Sans clutched the red scarf in his hands. "you came here, and then you turned around. you murdered everyone...was this just a kick for your sick sense of humor?" His eye flared blue.
"...I killed them all...in self-defense." Frisk slowly turned around. "They attacked me."
"you spared them, no LV to your name! you were good-"
"I'm still good." Frisk's grip around the knife tightened as small tears came to her eyes.
"you're a murderer!"
"I'm looking out for everyone." The child sniffled. "I'm looking out for whatever family and friends I used to have!"
"kiddo, what are you talking about?"
"All through this world, everyone's been talking about the destruction of humanity! My /race/! I thought I could make everyone see that we could be equal...but then...then..." Frisk sobbed and smashed the small casings that kept the SOULs hostage. They immediately swarmed to their hero. "Then I saw them." 
"the SOULs?"
"These were people Sans, like me. And now...they're dead. Knowing this...I can no longer spare you." The SOULs surrounded them and Frisk was slowly lifted into the air, their own SOUL ablaze. "I will protect humanity!"

Frisk Boss Battle, anyone?