"look, kid. we've all got a plan, so it'd be best if you just followed the rules,"
- UNDERPLAN Sans
The fallen human decided their plan was going to get them nowhere, stopping to consider the consequence.
The fallen human was saved by Asriel, and after a while, they developed a plan. A different plan. Accepting the potential consequences of their original, the fallen human whispered to Asriel their new plan: Wait.
The fallen human had devised a plan; wait until the humans fell and kill them. Asriel did not object and did as he was told, presenting the plan to his father and mother. Asgore hesitantly accepted the plan, and Toriel followed. After being pushed, they declared in a speech that all humans that fell will be killed.
Using buttercups and a bit of magic, Asriel and Chara stunted their age and growth, concerning their parents, but they thought nothing of it. For this plan to work, they needed an advantage. This stunt also stopped Toriel and Asgore from aging.
One by one humans fell and died. Covered in seeds, golden flowers blossomed here and there, overtaking the buttercups that grew as replacement.
By the time the final human fell, the first human had grown impatient.
"Look... I'm not supposed to tell you this, but..."
Asriel was taking a walk to get away from all the dramas and upon reaching the RUINS noticed an unconscious child lying in golden flowers and buttercups. He uncertainly wakes up the human, who freaks out. He calms them down and explains to them the basics; where they are and where to go.
Asriel smiles and turns around, but hesitates. He sighs and looks over his shoulder, explaining the human's' fate. He tells them it is unavoidable and that he's sorry. He starts to walk away but turns around. He looks down, engaging them in battle.
He explains battle mechanics to them the best way he can, and attacks them with a few gentle attacks. If the human gets hurt, Asriel immediately suspends his attack and heals the human.
Upon teaching them, Asriel wishes them good luck and gives Frisk a hug, walking away, and the human can choose to stop him. If they do, they are informed of Asriels' conflicting feelings, but regardless, Asriel walks away.