[In reference to sin as a “dangerous, predatory animal” in Genesis 4:7–“If you do what is right, will you not be accepted? But if you do not do what is right, sin is crouching at your door; it desires to have you, but you must rule over it.” NIV]
“God is saying . . . that if Cain sins, his own sin will eventually consume him. Sin is the suicidal action of the human soul against itself. The image also implies that sin isn’t simply a passing action; sinful actions create a dark reality in your life that stays with you. Sin creates bad habits; it creates distorted affections. These things control you, and you start to lose control of yourself. You’re surrendering to something that wants to kill you.”
Timothy Keller, Preaching: Communicating Faith in an Age of Skepticism. New York: Viking, 2015, pages 172-173.