Cain and Abel - theory that Cain is half human half serpent/ devil (that Eve lay with the devil or other fallen angel) therefore he is one of the Nephillim. He was impure that is why he could commit murder which was otherwise unknown to people at that time.
Cain killed Abel out of jealousy, and betrayed God. He committed what the Church calls Mortal Sin, and was not able to receive forgiveness for a sin of this magnitude. He is a perfect example of what to avoid in our lives. (Scripture)