Genesis 4 gives the account of Adam and Eve's sons, Cain and Abel.  

Adam had relations with his wife Eve, and she conceived, gave birth to Cain and said, “I have gotten a man with the help of the Lord.”  Then she gave birth again to his brother Abel.  And Abel was a keeper of flocks, but Cain was a tiller of the ground.  In the course of time Cain brought an offering to the Lord of the fruit of the ground.  Abel also brought the firstborn of his flock and of their fat portions. And the Lord had respect for Abel and for his offering,  but for Cain and for his offering, He did not have respect. And Cain was very angry and his countenance fell. The Lord said to Cain, “Why are you angry? Why is your countenance fallen?  If you do well, shall you not be accepted? But if you do not do well, sin is crouching at the door. It desires to dominate you, but you must rule over it.” Cain told Abel his brother. And it came about, when they were in the field, that Cain rose up against his brother Abel and killed him. The Lord said to Cain, “Where is Abel your brother?” He said, “I do not know. Am I my brother’s keeper?” And then He said, “What have you done? The voice of your brother’s blood is crying out to Me from the ground.  Now you are cursed from the ground which opened its mouth to receive your brother’s blood from your hand.  From now on when you till the ground, it will not yield for you its best. You will be a fugitive and a wanderer on the earth.”  Genesis 4:1-12 (Modern English Version MEV)

It may seem unfair that God looked favorably upon Abel's sacrifice and not on Cain's.  However, God was the One who performed the first sacrifice in Genesis 3:21.  He made coats to clothe Adam and Eve after they had sinned.  To clothe them, animals had to be killed.  This instituted the first sacrifice to cover man's sin.  Adam and Eve had tried to cover their own nakedness by sowing leaves together, but a blood sacrifice was required (Hebrews 9:22).

At the time of Cain and Abel, the only sacrifice that God had instituted was the blood sacrifice.  The acceptable means of sacrificing to God had been passed on to them by their father Adam.  Both of them knew they had to present an animal sacrifice to the Lord.  Abel followed protocol and offered the correct sacrifice from his flocks.  Cain, however, did not.  He brought a portion of the harvest from his fields.

We can tell quite a bit about Cain from his offering.  He had a heart of pride and did not want to submit to the approved way of the Lord.  He did not wish to give some of his harvest to his younger brother to purchase an animal to offer to the Lord.  In presenting an offering from plants and not animals, he was duplicating Adam's earlier effort to cover his own sin.  In essence, he was saying to God, "I'll do it my way, not yours."  It was a clear case of the sin of pride.  In addition, Cain chose to see God's rejection of his offering as a rejection of him as a person, rather than taking it as instruction on how to do the right thing.

God had warned Cain that he needed to prevent sin from dominating him.  Cain's pride blinded him, and instead of taking dominion over sin, he yielded to it.  In his jealousy and rage, he was ruled by his flesh and killed Abel, spilling the first human blood on the planet.  We should note here that Cain did the very thing that God was planning to do in the distant future, shed the blood of a human.  However, the only human blood that was ever intended to be poured out was the sinless blood of Jesus.  This premature shedding of human blood made this a serious sin indeed.  I am not sure about this, but it is even possible that Cain killed Abel out of spite toward God, as a way of sending the message, "You want a blood sacrifice, I'LL GIVE YOU A BLOOD SACRIFICE!"

One of the saddest aspects of this story is that there is no evidence that Cain repented.  I see no remorse in his conversation with God, only dismay at the harshness of his punishment.  It demonstrates the hardness of his heart and the extent to which he had allowed sin to dominate him that he apparently had no qualms about the murder he had committed.

We must regularly look at ourselves and examine our own hearts.  Are we allowing any of the pride that Cain had to enter our hearts?  Are we trying to do things our own way, or are we bowing to the ways of God?  Are we so focused on ourselves that we are willing to sacrifice the needs and wants of others to get what we want?  Are we allowing our flesh to rule us?

It is important to catch sin early to prevent it getting a foothold in our lives.  In our times of prayer, let us ask our Creator to show us any areas in which sin is crouching at the door, trying to dominate us.  We should ask Him to show us places of our hearts that need His cleansing touch.  It is only in allowing Him to direct us that our lives will go well. 

Proverbs 4:26 (KJV) says:

Ponder the path of thy feet, and let all thy ways be established.

Proverbs 16:3 (KJV) states:

Commit thy works unto the Lord, and thy thoughts shall be established.

Jude 10-11a (NLT) gives us a clear picture, telling of people who had fallen in the same trap as Cain. 

But these people scoff at things they do not understand. Like unthinking animals, they do whatever their instincts tell them, and so they bring about their own destruction. What sorrow awaits them! For they follow in the footsteps of Cain, who killed his brother.

We can avoid Cain's outcome by being God-ruled instead of flesh-ruled.  Let's humble ourselves before the Father daily for His cleansing and His leading in the paths of righteousness.