Family starts as the theme in Genesis 4:

Gen 4:1 Now Adam knew Eve his wife, and she conceived and bore Cain, and said, “I have acquired a man from the LORD.”

Gen 4:2 Then she bore again, this time his brother Abel. Now Abel was a keeper of sheep, but Cain was a tiller of the ground.

But from the beginning, we started killing each other

Gen 4:3 And in the process of time it came to pass that Cain brought an offering of the fruit of the ground to the LORD.

Gen 4:4 Abel also brought of the firstborn of his flock and of their fat. And the LORD respected Abel and his offering,

Gen 4:5 but He did not respect Cain and his offering. And Cain was very angry, and his countenance fell.

Gen 4:6 So the LORD said to Cain, “Why are you angry? And why has your countenance fallen?

Gen 4:7 “If you do well, will you not be accepted? And if you do not do well, sin lies at the door. And its desire is for you, but you should rule over it.”

Gen 4:8 Now Cain talked with Abel his brother; and it came to pass, when they were in the field, that Cain rose up against Abel his brother and killed him.

Gen 4:9 Then the LORD said to Cain, “Where is Abel your brother?” He said, “I do not know. Am I my brother’s keeper?”

Gen 4:10 And He said, “What have you done? The voice of your brother’s blood cries out to Me from the ground.

Gen 4:11 “So now you are cursed from the earth, which has opened its mouth to receive your brother’s blood from your hand.

This seems simply tragic, but it is important to note a few things. There were ideas in the common understanding of humanity long before the Law. Cain and Abel understood that it was good to come to God with a sacrifice.

Abel’s sacrifice came from the first born of his flocks. Cain’s sacrifice was a gathering of grain. Not a bad thing but certainly not the first fruit of what he grew.

God told Cain, “sin lieth at the door” in Genesis 4:7 when there was no specific command for Cain to break.

Cain’s jealousy led to revenge and the introduction of the land domain of war with a rock.

This begins the separation of the lineage of Cain from the lineage of Seth. That story is coming.