Alma 42:15

And now the plan of mercy could not be brought about, except an atonement should be made; therefore God himself atoneth for the sins of the world, to bring about the plan of mercy, to appease the demands of justice, that God might be a perfect, just God, and a merciful God. Also,

God’s plan of mercy couldn’t work unless someone paid for sins. So God himself paid for the sins of the world. This brought about the plan of mercy. It satisfied what’s fair. This way God could be both fair and merciful.

The plan of mercy couldn’t work without an atonement. So God himself atoned for the world’s sins to activate the plan of mercy, to satisfy justice’s demands, so that God could be both perfectly just and perfectly merciful.