When flowey pleads for mercy, does he really mean it?
if you want to go through a genocide run, the only way to spare flowey is to quit. if you want to kill him, simply just watch. but if you wnt to spare him, you HAVE to leave the game. it will count as a reset and send you back to the menu, oh btw, i know a easter egg where if you name yourself "gaster" it sends you back to the intro.