You probably know Cain as the bad guy who (for no good reason) killed his nice and kind brother Abel in Genesis, in the world’s first murder. Some of the unexplained problems of this account (such as why God accepted Abel’s sacrifice but not Cain’s, which is what pissed Cain off in the first place) are answered in the apocryphal book of Enoch.
Interestingly, the Book of Enoch also suggest another motive as well: Adam was planning to marry Cain off to the younger, ugly sister, while Abel got the better looking sister.
But as to the hard-hearted Cain, Satan came to him by night, showed himself and said to him, “Since Adam and Eve love your brother Abel so much more than they love you, they wish to join him in marriage to your beautiful sister because they love him. However, they wish to join you in marriage to his ugly sister, because they hate you.
Now before they do that, I am telling you that you should kill your brother. That way your sister will be left for you, and his sister will be cast away.”
This makes a lot more sense than the random act of violence found in the standard text. The book of Enoch actually goes further in characterizing Cain’s dark personality, and shows that he was constantly being manipulated by Satan. Plus, by introducing the sisters it answers one of the huge problems of the New Testament… which only shows Adam, Eve, and their two sons, Cain and Abel. If they were the only humans, who did they marry?