Since he remembers 'kill or be killed' was the rule of the Underground. Everything was simple at that time. He and his brother took care of things near Snowdin. By 'things' he meant humans, that eventually fallen into Underground, and by 'took care' he meant apply their rule. Kill or be killed.
But for them, strong monsters, no human could ever offer danger. He never felt fear of them or anything like it, only too bored or lazy to kill them by his own hands. Mostly he would just lead the pitiful humans to his brother. Sometimes the humans died due to his brother's traps and puzzles. Other times they fell by his brother's attacks. Anyway, seeing all that was funny.
Life was simple. Until the seventh human arrived, then everything started to change.
They just needed their SOUL to break the barrier and set monsters free forever. The first time he saw the human it was in one of those days he felt himself willing to kill. The human, strangely, had their face half-covered with golden flowers. He raised his hand to greet the human. They also lifted one of theirs, while the other was holding a yellow flower, a monster too apparently. With one single touch, an electric discharge from the device that he was holding made the human fall. Dead. He saw their SOUL, bright and red, came out of their dead body, pulsing with DETERMINATION.
But before he could take it, a strange feeling pulsed on his very own SOUL and he was patrolling the forest once again.
A reset. The first of many that would happen after that.
Every time the seventh human died, a reset occurred. The human knew what was happening and remembered everything, but anyways they always were kind to him, even after he killed or betrayed them. So much kindness began to make him think about the rule of the Underground. The human always acted with mercy and compassion, ever pure. He started to think twice about killing them. They didn't deserve. So, he broke the rule. He stopped doing so. He began to help them.
Life was simple before the seventh human. But, was hopeless e meaningless.
They taught him more than mercy and kindness, but joy, friendship, trust and, above all others, love, and fear. For a monster born and raised with other monsters, none of that mattered, only kill or be killed.
But, not for him anymore.
The last lesson was the most painful.
Sacrifice.
The seventh human sacrificed themselves for the sake of monster kind, to give freedom to everyone. It was their last action of kindness before the golden flowers consumed their body and strength. After that, not only him but all monsters learned goodness and mercy. None of them attacked humans again, living in peace with them now.
"What are you doing here, Sans?"
"I came to see you, sweetheart."
Frisk was sitting beside him, no golden flowers in their face, and showed Sans their best smile.
"You need to open your eyes, Sans."
"But…"
His eyes opened by themselves. Sans was sitting next to Frisk's grave, surrounded by golden flowers.
"If I open my eyes you disappear."
He laid down on the bed of flowers and cried.
Frisk taught Sans a lot of things, except one.
How to live without them.