"This is what the Lord says: 'For three sins of Moab, even for four, I will not turn back my wrath. Because he burned, as if to lime, the bones of Edom's king," NIV translation
The pattern of judgment that we saw in chapter 1 is coming closer to the home of God's people. Here, we see that the Moabites had committed a large number of sins but the charge presented here is very specific. The charge against them was that they sought revenge and were filled with a spirit of unforgiveness. If we, as Christians, have this same spirit or if we know that someone has not forgiven us, then we are called to go to them and ask for forgiveness. This is what Jesus told us to do in Matthew 5:23.