The 'mark of Cain' was the ancient equivalent to the modern English letters "H" and "S". IMHO, I think Cain was the very first member of the Humane Society. He probably became indignant at Abel for slaughtering innocent animals. Did Abel slaughter sheep just for the purpose of sacrifice? They didn't eat flesh at that time, right? So why did Jehovah want blood sacrifices so early in mankind's history? It seems to me that Abel was the first to shed innocent blood mentioned in the Bible. Just saying.
eyeuse2badub