We also have to keep in mind that all people are God’s creation. We are his to do with as he pleases. We all live and die with his fore-knowledge. The OT is mainly concerned with God’s justice, God’s honor, God’s triumph over evil nations fighting against the holy nation of Israel. It’s also about that holy nation failing God and sinning and in its turn experiencing the justice and wrath of God, as well as others.
God is not only loving, he is also just. Jesus satisfied the justice of God which is why we live in the age of grace instead of the age of justice. The OT is the prelude within God’s plan to satisfy his own justice. He did so through various means, most especially the Law of Moses. God’s wrath came down upon nations due to their hardness of heart, shown by their worship of gods besides himself. St. Paul wrote that they were without excuse since the evidence of God’s existence and power is in creation itself. IOW, they should have easily deduced that there is only one God and made every effort to worship him alone. That they didn’t shows they should have known better, especially since everyone in existence is the descendant of the same original parents to whom God spoke. Men invented gods to satisfy his own desires instead of worshiping the one true God.
The law and its sacrifices were only temporal, not eternal, which is why a divine person, Christ, was needed to satisfy it. As Paul says, “in the fullness of time God sent his Son”. Therefore the Incarnation brought about the end of the OT God holding the nations accountable in his justice and made possible the outpouring of God’s grace upon the whole world, not just upon Israel and those friendly to it.
As I think I mentioned, this is a complex topic. One that cannot be answered in a nice glib sentence or two. It deals with history and how God was leading up to the Redemption of all men, preparing the world for it through the Law of Moses and by keeping Israel pure and true to its purpose, which sometimes entailed fighting pagans and heathens in order to do that.
I can’t think of a book on the topic that would help your friend understand all this (hopefully better than I can explain it here). My college days are far behind me, so I cannot make any recommendations, but I suggest you look into it.
