Q) – If God knows everything all at once, why was he violent in the old testament, but wants peace in the new?
A) – This is a great question.
We know that God Himself is unchanging.
Hebrews 13:8 – Jesus Christ is the same yesterday and today and forever.
Malachi 3:6 – “For I, the LORD, do not change;
Therefore, we must look at it, not as if he changed his mind, or changed his personality as we tend to do as we grow older, but instead we just see Him expressing Himself differently to different people at different times.
For example, we see evidence of Gods wrath and mercy in both the Old and New Testament times.
Jesus, who is known for His mercy, grace, and forgiveness got “angry” and started flipping tables and whipping people when he went into the temple square and saw the Jews extorting each other to profit on worship. (John 2:13-17, & Matthew 21:12-13)
And the Heavenly Father, who is known for his wrath in the Old Testament shows his mercy and forgiveness time and time again to the nation of Israel, and to other nations that offended him, like the Ninevites who repented after Jonah preached repentance to them. (Jonah chapter 4).
The examples of Gods mercy in the Old Testament are too numerous to list though. But I think God became most well known by us today as the vengeful God of the Old Testament because from the Old Testament times, we have many records of God dealing with sin severely. The reason for this was always the same, which was to show mankind how wicked we are in our flesh, and how impossible it is too meet God’s holy requirements without the help of a savior.
Each time we read about God dealing with sin severely, we should remember that the cost of any sin is absolute death, which is just as much the same today as it was thousands of years ago. But our God loves all mankind, from past, present and future, enough that He would rather die in our place then to see us pay the high cost we deserve to pay.
I hope this helps to answer your question.