The Cross in Genesis 4.7: Cain Rejected and Rejecting

Is sin crouching at the door, ready to pounce on Cain in Genesis 4:7, or is a “sin offering” lying down ready to extend to Cain the very mercy of God?
Is sin crouching at the door, ready to pounce on Cain in Genesis 4:7, or is a “sin offering” lying down ready to extend to Cain the very mercy of God?