Why does the Lord send an army of an evil nation to destroy one who has fallen from spirituality? Because the Lord does everything for the good of man, in different ways, I believe that even warfare is an attempt to redeem people, on both sides. A fallen nation could remember God and return and repent, while the advancing army could also see their own wicked ways and repent as well. In every instance, the Lord could have sent fire from heaven, but instead sent people.
See also Helaman 8:7, Alma 24:25-27, Isaiah 7:17
There are a lot of scriptures where the prophets command people to repent. It can seem a bit brash at times, out of context, the way that the message is so forcefully given. It is in cases like this that it is good to remember Heavenly Father as a person, and who is approaching His children's situations in a very personal way.
Calling people to repentance isn't just a commandment to end wickedness, it's a promise and a plea that is instructing them how to exit the horrible situations in life they find themselves stuck in, spiritually, socially, and so on. It is the invitation to a better life, and the quickest way and most effective path to get there.
Without the Atonement
What would it be like to not have the Atonement? I thought of a situation - what if there are two close friends who have a falling out? Without the opportunity and power of restoration, that relationship would always be fractured. There'd be no way to back to the way things were. You would be cut off forever, living with regret and sadness.