Since the fall of man, when disobedience first occurred and the knowledge of evil came into being, this race has been hell bent. Adam and Eve felt the sting of the decision they made for us all, not only in the curses they earned, but most potently when their firstborn son plotted, and then killed, his brother. You can read about it here, if you like.
Isn't the first thing detectives look for when investigating a murder, the motive? But Cain had no insurance settlement to gain. There was no woman they were fighting over. Just like you and me, he only needed a corrupted heart to spawn this original homicide. If anything, we might point to a mere "sibling rivalry" to explain Cain's evil deed. In conversation with Cain, before the first murder, God warned Cain about the beast of Sin that was "crouching" at his door.
Then the Lord said to Cain, “Why are you angry? Why is your face downcast? 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.” From Genesis 4, linked above.
Murder and mayhem are all too familiar to us simply because it doesn't take much to lead us astray. But we have a way out! God's warning to Cain also showed the way. Cain knew God directly, and talked with Him. Through this relationship, Cain should have been able to "rule" his temptations. Because of the grace and mercy of God, established and offered the very same day Sin showed its ugly face in the family picture of man, Cain could have done better. We, through a relationship with God established in Christ, are also enabled to overcome the tyranny of Sin!
Don’t you know that when you offer yourselves to someone as obedient slaves, you are slaves of the one you obey—whether you are slaves to sin, which leads to death, or to obedience, which leads to righteousness? Romans 6:16
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