Why did Cain kill his brother Abel? What was his reason for doing so?
A Costly Sacrifice
When David went to make a sacrifice “Araunah looked down, he saw the king and his servants coming on toward him. And Araunah went out and paid homage to the king with his face to the ground” (2nd Sam 2:20) and Araunah asked, “Why has my lord the king come to his servant?” David said, “To buy the threshing floor from you, in order to build an altar to the Lord, that the plague may be averted from the people” (2nd Sam 2:21) so Araunah said, “Let my lord the king take and offer up what seems good to him. Here are the oxen for the burnt offering and the threshing sledges and the yokes of the oxen for the wood. All this, O king, Araunah gives to the king.” And Araunah said to the king, “May the Lord your God accept you” (2nd Sam 2:23-24), but King David said, “No, but I will buy it from you for a price. I will not offer burnt offerings to the Lord my God that cost me nothing.” So David bought the threshing floor and the oxen for fifty shekels of silver” (2nd Sam 2:24).
A Better Sacrifice
After the fall in the Garden, the rest of mankind inherited the curse of the fall. Nothing would come easily any more like it did in the Garden because sin had separated mankind from God and has shattered the intimate fellowship that Adam and Eve  previously had. Now, the “sin gene” would be in every person ever born, up until Jesus Christ would return again. Only Jesus Christ, as the God-Man, never sinned. He alone was without sin. It wasn’t long before mankind started to bring offerings, and it was in the first generation that Cain and Abel began to bring offerings unto the Lord (Gen 4:3), but why did they sacrifice? It must have been revealed to them that God demands a sacrifice for the sins of mankind, and even though Abel was a righteous man, he, like the rest of us, sinned (Rom 3:23), so Abel made a costly sacrifice to God. As a shepherd, he raised sheep, probably more for sacrifices than for anything else, and so Abel brought the “fat portions from some of the first-born of his flock” (Gen 4:4) and God looked with favor on Abel’s sacrifice. The author of Hebrews wrote, “By faith Abel offered God a better sacrifice than Cain did” (Heb 11:4), but why was Abel’s sacrifice better than Cain’s. Cain brought “some of the fruits of the soil as an offering to the Lord” (Gen 4:3).
Unacceptable to God
When we give things that don’t cost us much or they come at the expense of someone else, God is not impressed or pleased. For example, one person who has great wealth might give $20 to charity, but the person who has little gave 10 dollars, even though they could barely afford it. The rich man gave a greater amount, but the poor person actually gave more because they gave out of the little they had. The rich man gave out of his abundance. Which was costlier? Clearly, it was the one who had little because they gave out of their poverty. Comparatively, the rich man gave only a drop in the bucket. It’s exactly like the widow’s mite that Jesus spoke about. Others gave a lot, but they did it to be seen by others, but no one watched the poor widow (but God) who gave all that she had. That’s because “they all contributed out of their abundance, but she out of her poverty has put in everything she had, all she had to live on” (Mark 12:44). The widow’s mite was acceptable to God because it was very costly to her, but the rich man’s meager twenty dollar bill was an insult to God Who gave this rich man all that he had, because it is God “who gives you power to get wealth” (Deut 8:18a). The Apostle Paul asks more than just the Corinthians; he asks all of us, “What do you have that you did not receive? And if you did receive it, why do you boast as though you did not” (1st Cor 4:3-4)?
Why Cain Murdered Abel
I believe God accepted Abel’s sacrifice because Abel knew that sin was costly and how offensive it was to God. Abel knew that sin required the life on an innocent victim’s shed blood and why he offered sheep. Abel likely raised sheep for sacrifices, although their wool would certainly be useful, but Cain had a different sacrifice. He gave the “fat portions from some of the first-born of his flock” (Gen 4:4), meaning the very best of the sheep. Think of the slaughtering process. How much toil and sweat, blood and guts, and hearing the animals scream and seeing them die. God’s point is; sin is very costly…meaning, it is very bad. On the other hand, Cain only offered “some of the fruits of the soil as an offering to the Lord” (Gen 4:3), which didn’t require much sacrifice on his part. It says “some of the fruits” which may mean, not the choicest of the fruits. There is also the idea of works entering into it. Cain worked the garden to produce the fruits while Abel offered what God had blessed him with, and that is sheep, and again, fruits cannot cover sin. It takes the sprinkling of the blood on the mercy seat, not carrots and beans. When God accepted Abel’s sacrifice but wasn’t pleased with Cain’s, Cain became jealous of Abel, and I believe that’s why he murdered him, as it says, “Cain spoke to Abel his brother. And when they were in the field, Cain rose up against his brother Abel and killed him” (Gen 4:8). The Apostle John answers the question as to why Cain murdered Abel,  as he wrote, “Do not be like Cain, who belonged to the evil one and murdered his brother. And why did he murder him? Because his own actions were evil and his brother’s were righteous” (1st John 3:12).
The Apostle John warns all of us to “not be like Cain” because Cain’s sins were not covered, and he didn’t offer much of anything to God. In his heart he became angry, jealous, and had, in effect, murdered Abel in his own heart, long before he did it in the fields. “Some fruits” is not going to do it! It took the blood of a lamb because God wants us to know that sin is deadly serious and it requires someone or something’s sacrifice (Rom 6:23b). Fruits cannot cover sins like the blood of an innocent animal can, although these only covered sins and never took them away. It took the precious blood of the Lamb of God to take our sins away. The Apostle Peter wrote “that you were ransomed from the futile ways inherited from your forefathers, not with perishable things such as silver or gold, but with the precious blood of Christ, like that of a lamb without blemish or spot” (1st Pet 1:18-19).
Read more about Cain and Abel here: Cain and Abel Bible Story 
Resource – Scripture quotations are from The Holy Bible, English Standard Version® (ESV®), copyright © 2001 by Crossway, a publishing ministry of Good News Publishers. Used by permission. All rights reserved.