Genesis 4:6-7 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 master it."
God always gives us, leaves us a way out of temptation, a way out of sinning.
1 Corinthians 10:13 No temptation has seized you except what is common to man. And God is faithful, he will not let you be tempted beyond what you can bear. But when you are tempted, he will also provide a way out so that you can stand up under it.
God was warning Cain, telling him to think about his actions, get control over his feelings. He lovingly laid it out.
If you do what is right, you will be accepted (obey me, God)
But if you don't do what is right (choose not to listen to me, God, choose not to obey)
Sin is crouching at your door (the opportunity, desire to do wrong is right there)
It desires you (It wants to consume you, it wants you to disobey)
But you must master it (you can master it, but you have to do the work to do so. You must get in control of it rather than it be in control of you).
Of course Cain does not listen and we have the first murder on earth.
One generation removed from Adam and Eve living in the garden and Cain has so much anger he wants to hurt his brother, Abel. We don't know if they had seen animals dead by now, chances are probably if they killed them for food.
But this was the first man killed. How it must have surprised them. How they must have grasped their own mortality. How it must have shocked Adam and Eve.
The Bible doesn't say and we can just draw our own conclusions, but I would imagine Abel was mourned by Adam and Eve, like we mourn those we love who have died. They had no idea or hope that they would see Abel again . At least us who believe in Jesus know we will see our loved ones again if they too have faith in Jesus.
It does fascinate me with the story of Adam and Eve, Cain and Abel.
But God does make beauty out of ashes, even here, because years later one of Cain's relatives is Noah, a man who loved God and obeyed him.