@Gormin probably the old demon lord isn't really a bad person, the kingdom and the people there is probably already corrupt beyond saving, and by chance there's a hero full of justice and strong enough to kill her.
So maybe killing herself, resetting the kingdom by destroying it, and leaving the next demon lord on a good person's hand is the demon lord's roundabout way to safe the demon side and maybe potentially making peace with the humans.
But again, this is just my theory. The author probably didn't give a damn about the reason why this happen.
And now I feel like a fool for typing this text wall.