The 'coals of fire' is a picture of conviction — the discomfort of unearned kindness that awakens a person's conscience to their own wrong. But the verse also gives a clear motivation: God Himself rewards acts of love toward enemies. Justice is not our job — God handles it.
Memory phrase: God rewards kindness to enemies.
Talk about it: Why can you trust God to take care of things when you choose to be kind instead of getting back?
Prayer: God, help me trust You to reward me for doing right.