After Frisk left the underground (neutral ending), Toriel returned as queen and ruled the kingdom in Asgore's place, and also introduced other policies... But those policies faced opposition from all the monster residents, who were very depressed and disappointed after Frisk left and left behind many losses, they want to kill all humans who fall here in the future to avoid disaster.

(At this point it was long after Frisk left, you were a human [regardless of age] who fell underground and was brought before Toriel for judgment 'don't worry, Toriel won't kill you')