Cain was the Product of the Knowledge that Satan Beguiled Eve with. Satan made Eve to desire the same desire that Satan had from his Fall; which is to Be like GOD to be able to create.
Satan did not have this power and still does not have this power. It is one of the reasons he Hates Human Females and why he seeks to have them fall under his rule.
Eve then took the Knowledge and Shared it with Adam and he whole heartedly excepted it as truth. This knowledge was that of the wedding dance (sex) taught to her by Satan (sex). Yes, Cain is Satan's offspring. As is Able the offspring of Adam.
Eve Dance with them both and conceived from them both a Man child or an prince to their kingdoms. They were conceived the same said day. Two separate eggs and two separate Sperms which created two Separate Humans. Cain and Able were Twins by two different Fathers. They both came of age they same year and both made offerings to GOD the Father at the same season.
Satan's son Cain could have been the first Hi-bred an Adam's son Able the first an only innocent Human soul. Satan an Adam were not human. but when Adam fell he became So as Satan did Not.