Once his friends had disappeared, Sans let his anger show. His eyes glowed blue. He felt his hatred overwhelm him as he stared at Flowey. He couldn't help it. He remembered everything now.
Before Frisk faced Sans the first time, Flowey told him everything. He was the one who had played with their lives. He was the one who had killed simply because he was bored. He was the one who convinced Frisk that killing was all he could do. Ironically enough, he was also the one who snapped Frisk out of his genocide.
Sans could see Frisk's memories as well as his own and began reliving them.
Frisk thought he was avenging his parents, but he started to doubt, and those doubts grew stronger as he grew closer to the Judgment Hall and listened to Flowey's story. When the flower finished talking was when Frisk started coming to his senses.
When Flowey finished talking, Frisk stood in place staring at the dust on his hands as tears began blinding his vision.
"Chara?" Flowey had asked. "What's the matter, Chara? Let's finish what we started. Then, we'll take the world down just like you wanted."
Frisk sprinted away from Flowey as quickly as he could into the Judgment Hall.
"Chara? !" Flowey yelled after him.
Frisk kept sprinting until he reached the Judgment Hall where Sans was. Sans remembered how he looked. Tears were going down his face, but Sans had no sympathy for Frisk yet. He had killed his brother and everyone else.
Sans began his spiel, and Frisk didn't say a word until he mentioned the promise he made to Toriel.
"Do you really feel bad?" Frisk asked desperately finally looking Sans in the face. "Do you really feel bad about breaking the promise?"
"What?" Sans asked incredulously. "Of course, I do! I didn't want this to happen! You've given me no choice, kid! You've murdered her and my brother and everyone else so I have to…"
"Oh, my God," Frisk said quietly while dropping his knife on the floor. "What have I done? WHAT HAVE I DONE?! I'M A MONSTER."
Frisk fell onto his hands and knees and started sobbing uncontrollably. Sans stood there unable to comprehend what was happening.
"I'm so sorry, Sans," Frisk said between sobs. "I thought…He said it was kill or be killed. I thought you felt nothing. I thought you were all bad! I thought I was doing the right thing! Oh, my God! I'm going to hell."
Sans still felt little sympathy.
"It's too late for that, kid," Sans said. "You've ruined everything. You've played with our lives too much for me to feel bad for you now. The best thing you can do is let me kill you or reset…"
"Reset?" Frisk asked looking up and stopping his sobbing for a moment. "Like turn back time?"
Sans was confused. This kid had to know how to reset.
"Yes," Sans said. "You turn back time with your determination! Do you mean to tell me you don't know what that is?"
Frisk picked up the knife he dropped, rolled back his sleeves, and said, "My parents told me we could use determination to make miracles happen. All we needed was tears, sweat, and blood."
Sans asked, "But if you're not the one who has been resetting time, then who…?"
The tile broke apart and Flowey came out of the floor.
"What the hell…?" Sans asked. "Who are you?"
"Chara, what are you doing?" Flowey asked. "What aren't you killing him?"
Frisk moved his hands in a waving motion and started pulling his tears and his sweat out in front of him.
"Chara?!" Flowey yelled. "We're so close! You've done what I haven't managed to do in years in all my resets! Why are you turning back?!"
"That was you?!" Sans asked.
"Chara, you're blowing everything!" Flowey yelled more angrily now. "How could you?! I've been waiting all this time for this chance! We were gonna destroy this boring world! Why are you doing this?!"
"My name's not Chara!" Frisk yelled. "It's Frisk, and I'm going to make things right! You lied to me! You let me think they felt nothing!"
Flowey was visibly crushed, but very quickly, his sigh curled into a dark smile and he started laughing.
"It looks like I really was fooling myself this whole time," Flowey said with a smile. "But I'm not the only one. You fooled yourself, kid. I never told you they were evil. You came to that conclusion. All. On. Your. Own. You were so focused on carrying out your little mission that you didn't even think about that. Did you? Now, you have blood on your hands. Or should I say dust?"
"You, son of a bitch!" Sans yelled pulling out his Gaster Blasters to fire at Flowey.
"Don't focus your energy on me, smiley trash bag," Flowey said. "Why don't you focus it on the one who killed your pathetic little brother?"
A bright glow lit up the room. Frisk had cut open the vein on his left arm, and the blood was flowing through the air and glowing with its magic. It surrounded Frisk in coils as he breathed hard and focused his energy as well as he could until a reset button appeared magically before him.
"Frisk?" Sans asked.
"I'm going back, Sans," Frisk said with a hopeful smile. "I'm going to fix everything. I'll bring Papyrus back and everyone else back and try to give you the happiest ending I can."
Sans teared up and said, "Thank you."
"No!" Flowey screamed. "NO! We were so close! I'll make you pay, you little brat! I'll kill you!"
"Make another move and you're dead," Sans said surrounding him with bones.
Frisk pressed the button and the Judgment Hall filled up with light that consumed the room while Flowey screamed in the background. The next thing Sans remembered he was at his post in Snowdin.
Sans continued glaring at Flowey in absolute hatred as he remembered everything that Frisk endured after that.
The flower tormented him horribly. Every mind game, every manipulative word, every insult wore Frisk down until he could not endure it anymore and wanted to take his own life. That was Flowey's plan. He wanted to wear Frisk down until he could take his soul for himself and never bring about the happy ending he desperately wanted.
Sadly, Sans could not do what he desperately wanted.
He sighed deeply and said, "I hate making promises."
Sans tried picking up the flower by his soul, but nothing happened. He tried again but still nothing. Sans finally waved his hand to check the flower's stats.
"That's right," Sans said to himself. "You have no soul. So, how did I move you before?"
Sans didn't feel like wasting too much time thinking about it, so he picked up Flowey with his right hand and short cut away.