@Veritas
I don't have answer for you as to why the God of the OT looks so different from the God of the NT that we see in Jesus Christ.
There is a Christian preacher and scholar who published a couple books on the subject in 2017, one academically inclined and the other for laypersons. His name is Greg Boyd. The book is called "Cross Vision: How the Crucifixion of Jesus Makes Sense of Old Testament Violence."
Greg has blog with articles about his view. He might surprise you. I have the book (for layman's version) but haven't read it yet. My loss, otherwise I could have given you an answer immediately.
Here are a couple of links in his blog on the subject:
There are two basic questions that help us to interpret what is going on in the violent portraits of God in the Old Testament, as I propose in Crucifixion of the Warrior God. The First Question: What does the “God-breathed” revelation of the cross teach us about the nature of God’s “breathing”...
reknew.org
In the second volume of Crucifixion of the Warrior God, I introduce how four dimensions of the revelation of God on the cross (as introduced in this post) lead to four principles that show us how to unlock aspects of the OT’s violent divine portraits and thus disclose how a given portrait bears...
reknew.org
If you aren't interested in reading these articles, then you will have to wait for my answer when I finish reading his book.