Once upon a time, there was a little monster child named Asriel.
Asriel, like all the other monsters, lived in the underground, sealed away from the surface and the sun.
One day, something amazing happened; Asriel met a human. It had fallen down from the surface world and was trapped in the underground like the monsters.
Asriel and the human became best friends. They played, exchanged secrets and did everything together. The human even knew how to free all monsters from the underground and get them all to the surface. And he wanted to do it with Asriel.
But the human's plan failed, and both Asriel and the humans died.
All because Asriel was an naive idiot.
But Asriel didn't remain dead, as a scientist named Alphys decided to bring him back to life. Alphys took Asriel's remains and injected them into a little flower and then gave the flower the will to live.
And because of that, Asriel was now alive again as a little golden flower.
But Asriel didn't like being a flower. It was weird for the little boy not having arms and legs anymore. And worst of all; Asriel no longer had a soul, making it impossible for him to feel emotions. He could do everything a living person could do; talk, think, move... but not feel. No matter what he did, he couldn't feel anything for anyone, not even for his own parents. They just annoyed him.
So Asriel decided to commit suicide, ending his pathetic existence.
But as Asriel could feel that he was about die, he was frightened and desperately wanted to live, to evade dying.
And so he did.
Instead of dying, Asriel found himself in the garden where he had woken up for the first time as a flower. He discovered that he had traveled back in time.
Intrigued, Asriel killed himself again. And then, he did it again.
Asriel discovered that he had the ability to reset time purely by determination, by simply wanting to live hard enough. So Asriel decide that he was no longer Asriel. He was someone new, something new. Something better. He was Flowey. Flowey the Flower.
With the power of his determination, Flowey decided to help everyone in the underground, to do good deeds with his powers. And he succeeded in fixing everyone's problems to perfection. Whenever he did something wrong, all he had to do was reset time and do it again until he got it right. Al the monsters were happy, thanks to him. Even though he couldn't feel happiness, he still felt satisfaction for his accomplishment. But he eventually grew bored, and decided to reset the timeline, doing it all over again, only a bit different to see how people would react this time. Eventually, he had done that so many times that it bored him. So Flowey decided to do something entirely new to satisfy his curiosity;
He killed someone. Just to see what it was like.
After all, with the ability to reset time, it wouldn't really mean anything. He would simply reset, and that person would be dead no more.
Killing was a new experience. A new game. And without feelings, Flowey didn't feel guilt about taking a life. So he kept killing, he kept resetting and went on like that over and over again. Flowey found it surprisingly satisfying and entertaining to kill. Especially after gaining someone's trust. To see their happy faces turn to shock as they realized that they had been betrayed. To become a symbol of fear in the underground. To watch everyone either run of foolishly trying to face him. Their resistance was futile since killing Flowey was meaningless. He just had to reset, and learn from his mistakes, and go on killing with the knowledge he had gained his defeat.
He was practically a god.
Flowey saved everyone and killed everyone. He won every game and lost every game, he read every book and burned every book.
And eventually, Flowey grew bored once more. Having tried everything, Flowey decided to reset and wait, believing that if he waited long enough something new would happen.
And then, something new DID happen.
A human fell to the underground.
Flowey found great satisfaction in having found someone new to mess with, and decided to lure the human into a trap and then kill them.
But the human didn't fall for Flowey's trick. The human wouldn't believe that Flowey only wanted to help. At first, this annoyed Flowey... until he realized something odd about the human. The way the human acted, the way they looked at him. Like they knew what to expect. Like they had... seen it all before.
The way he had seen things before that no one else had seen.
After trying to reset time, Flowey discovered that he couldn't. something prevented him from doing so.
It had to be the human. They had to have the same power. And apparently, their will for this world was overriding Flowey's.
And Flowey was so excited. This was an entirely new sort of game. One he intended to win.