In my view of Scripture and theology, those themes in the Old Testament which appear to be contradictions are generally explicable in terms of covenant. God treats some people groups one way, and other people groups another way because of his different covenant relationships with them. He is much more merciful, patient and loving to those in covenant with him, even if the individuals in the covenant people group are not saved. Covenant explains much apparent inconsistency.

Similarly, covenant can often explain the fact that God sometimes changes the way his treats the same group of people. Because all covenants are conditional (contrary to the more popular but somewhat dated view that some are conditional and some are unconditional), God may change his attitudes, actions, offers, threats, etc. toward people when they change their behavior toward him. That is, if he threatens a curse, and those against whom he has threatened it repent, he may well decide not to curse them, and might even bless them (compare Jonah 3). In the same way, his blessings are generally contingent upon continued obedience. Jeremiah 18 lays out these general parameters. We also have a good article on the site which deals with this issue: Historical Contingencies and Biblical Predictions.

When it comes to people who are faithful to God, true believers, God tends to be even more merciful to them than he is to those who are simply part of the covenant people. To put it a slightly different way, he is more inclined to bless those who are accounted covenant keepers than he is to bless those who are accounted covenant breakers.

When we find things in the Old Testament that are not explicable in terms of covenant, they can usually be explained by the freedom that attends God's sovereignty. Continuing the idea of contingent blessings and curses as found in Jeremiah 18, it is important to remember that God is not bound by human behavior. He might change his mind when people change their actions, but he is not bound to do so -- the "who knows" verses of the Bible make this clear (e.g. 2 Sam. 12:22; Joel 2:14; Jon. 3:9). God is free to do whatever he wants to do whenever he wants to do it. His covenant is a good indicator of his heart and will, but it does not reveal everything. God has great freedom to act within the parameters of the covenant. One good example of this is Job: Job was a righteous man, faithful to God, yet God allowed him to suffer greatly.

Another thing to remember is that God considers a longer timetable than do most people. As in the case of Job, God allowed terrible things to happen for a while even to one of the most righteous men ever to have lived. Nevertheless, he eventually blessed Job in this life. Of course, this is not always the case -- sometimes people don't ever get blessed in this life. But God's timetable extends beyond this life. He will ultimately bless all those who are accounted covenant keepers, even if they know nothing but apparent curses in this life (e.g. Job 19:26). The apparent mistreatment of those who are faithful to the covenant is something of a mystery (e.g. Job 42), but it works for the good of those who suffer and is motivated by God's love (e.g. Prov. 3:12; Ps. 119:75).