Hi
'i' understand Adam & Eve as having two sons at the start. Both were the children of God in Obedience untile maturity. Then we see in Gen. 4:7 that Cain was disobedient even after God spoke directly to him. I find this was mature total rebellion, & Cain was now found sining against the Holy Ghost and was now left to be used of satan with a new 'desire'.
I see this as satans first full/fledged convert (or son of sinful mankind) & he was the devils first killer. (of his brother) And around 120 yrs. in the future Seth came on the seen to replace Abel as the obedient son of God. (as in man)
What is of interest is that it was Adams 'first' born that rebelled against God as well! Eccl. 3:15 Surely the problem seen here was that of maturing the wrong way! Phil. 4:13 + 2 Cor. 12:9