This destruction and this condemnation will not happen because Yahweh wishes it but rather because of personal choice. Individual agency will still work—but with terrible results. Those who would perish will do so “unto” themselves—meaning that they have chosen to perish. Their own decisions have led to this result.
Even with this dire prediction, Samuel holds out hope. They are still free to choose, they are still “permitted to act for [them]selves.” They can still repent. If they repent, the Messiah’s atonement will save them. If not, they perish, “unto themselves.”