Ultimately, the Old Testament (in my view) is quite a mess. God seems very… fickle and petty?

If you do a deep dive into Genesis 6 (the Book of Enoch helps with this greatly), a very different picture emerges.

Genesis 6:1-4

1 And it came to pass, when men began to multiply on the face of the earth, and daughters were born unto them,

2 That the sons of God saw the daughters of men that they were fair; and they took them wives of all which they chose.

3 And the Lord said, My spirit shall not always strive with man, for that he also is flesh: yet his days shall be an hundred and twenty years.

4 There were giants in the earth in those days; and also after that, when the sons of God came in unto the daughters of men, and they bare children to them, the same became mighty men which were of old, men of renown.

So the sons of God AKA angels mated with the daughters of men, which resulted in the birth of giants and demigods. This act corrupted the DNA of mankind.

Genesis 6:5-7

5 And God saw that the wickedness of man was great in the earth, and that every imagination of the thoughts of his heart was only evil continually.

6 And it repented the Lord that he had made man on the earth, and it grieved him at his heart.

7 And the Lord said, I will destroy man whom I have created from the face of the earth; both man, and beast, and the creeping thing, and the fowls of the air; for it repenteth me that I have made them.

 The effect of man’s sexual immorality was staggering since it altered the race. The offspring of this union between human females and the fallen angelic beings were physical giants. These children were not fully human. They grew enormous in stature with great intellects and were called “men of renown.”  

 This cohabitation between humans and the spiritual beings was not isolated to a few individuals but became widespread throughout the earth. 

 This cohabitation was part of the reason God’s heart was grieved with mankind and triggered the flood as judgment to wipe out this corrupted race of humans. In modern science, the biologists would identify what happened to mankind as altering the human DNA. 

 The DNA of man was changed by this union and some men were no longer fully men. They were a hybrid.

https://www.defendproclaimthefaith.org/DaysNoah.htm

Genesis 6:8-9

8 But Noah found grace in the eyes of the Lord.

9 These are the generations of Noah: Noah was a just man and perfect in his generations, and Noah walked with God.

The Bible describes God’s selection of Noah in order to preserve mankind. Noah qualified because he was “perfect” in his generation. This does not mean he was a morally perfect human, but that he was genetically perfect in his lineage. He had perfect human DNA and was not corrupted by the sons of God.

This fickle and petty God, as you refer to him, waited until the last 8 purebloods were alive before He shut the doors of the Ark and destroyed the planet with the Great Flood.

Now, after the Great Flood, we discover there was a revival of the Nephilim. They settled in the land of Canaan after God promised that land to Abraham and his descendants. The tribes God ordered Israel to destroy utterly and completely were not fully human.