Out of all the sins the Lord could have listed as his reasons for destroying so many and so much, the one he lists really sums the rest of them up, and that is the killing of the prophets, those he placed on the earth to call the people to repentance. The greatest destruction took place in the land northward at the city of Jacobugath, where Jacob was king over all of the people who were of the secret combinations. If you're wondering how bad these people were, the Lord says their sins and wickedness were above all the wickedness of the whole earth. The places that were completely destroyed the Lord says were places where "there was no righteous among them."
Then he turns to those who were spared because they were more righteous and asks them what I think is a very loving and tender question, "Will ye not now return to me, and repent of your sins, and be converted, that I may heal you. That is who he is, the source of all healing and there is no better source we can turn to when we are in need of being cared for. He asks us to first do two things, repent and be converted. I think of Alma's question, "If ye have experienced a change of heart... can you feel so now?"
To me, another important point in this chapter is verse 20 where the Lord says that the Lamanites were baptized with fire and with the Holy Ghost "and they knew it not." We have no idea how many times we have been influenced by the Holy Ghost. Some day in the future, we will learn just how much the Lord has been involved in our lives. When people come to our aid or support, when good things happen, we often assume that it just happened naturally or it was by our own effort. I don't believe that anything good that happens is ever totally devoid of the influence of the Spirit.
The three days of darkness obviously coincide with the three days the Lord's body laid in the tomb owned by Joseph of Arimathaea. With his resurrection, the light of the universe burst forth again on all of the world. For 400 hundred years, the Nephites will bask in this light. But in the rest of the world, it will be rejected. The light of Christ is both symbolic and literal. Symbolic in that it represents the light that has obliterated the darkness of death through his resurrection. Literal in that, as it says in the 88th section of the Doctrine and Covenants, it is the light by which all things are created, upheld, and by which all things operate.