The Spread of Sin.
Human beings were not created by God for sin; unlike our physical bodies which possess organs like the liver to process various toxins, we lack the capacity to process sin. When Adam let sin into human experience, he unleashed a force against which we cannot win with our own resources. The impact of sin is seen in the proximity of Genesis 4 to Genesis 3: within one generation, people were already killing each other. Sin is deadly and destructive, separating people from God, alienating us from each other, and introducing chaos into God's created universe.
There is much to learn from Cain; although guilty, he never repented, he never humbled himself before God, he never asked for mercy. He was sorry - not that he had offended a holy God, but that he got caught; he couldn't get away with what he wanted to do.
And yet, although he deserved much worse, God mercifully built in restrains to limit the impact of sin. He acted to slow its growth. But there's more: the blood of Abel points forward to the blood that Jesus would shed on the cross, blood that is the ultimate expression of God's mercy. For on the cross Jesus bore the penalty that should have come to us. As the perfect older brother, Jesus protects us and keeps us and brings us safely home.
Like Cain, we've all done stuff that's wrong. That's what sin is. And the Bible is absolutely accurate: all have sinned; we have inherited Adam's nature and his rebellion is built into the fabric of our being. But the good news is that God has intervened through Christ to save us. The good news is that God has mercy on us. He invites us to repent - to turn from sin and to turn to him - not because we've been caught, but because He is holy, we are not, but he has given us mercy, and through Christ we can be forgiven. That's why it's good news.