And Kain brought a sacrifice to G-d from the fruit of the earth. And Hevel also brought a sacrifice from the first born of his flock and from their fat, and G-d had respect for Hevel and his sacrifice. To Kain and his sacrifice he did not have respect. (Bereshit 4-4)We are driven to ask several questions on this story which ended with the tragic death of Hevel and the subsequesnt punishment of Kain that he must spend the rest of his life wondering.
Why did G-d like the person Hevel and appreciate his sacrifice and dislike the person Kain and his sacrifice? What drove Kain to kill his brother?
For the sake of brevity and the fact that Shabat is starting in less than two hours we will let these two questions suffice.
Perhaps the answers to these questions are one. Hevel was a good person whose activities were dedicated to G-d and was humble. Kain was the opposite. Our rabbis have said that Kain brought his sacrifice with flax seed. Flax seed is not a very honorable item. It seems that he was also filled with himself and did not have the brains to even answer G-d when he told Kain:
Why are you angry and look depressed? If you try do well you can exceed, If you do not try to do well then sin awaits you and he will desire you and you will rule over him.Before G-d addressed him Kain was already too filled with himself to realize how abhorrent he was and why his sacrifice was not aceptable. We know that a sacrifice given by a non-repentant person is a waste of time and abhorant to G-d. After G-d addressed and even told him that he can improve himself Kain went over to Hevel and butchered him.
Being filled with himself Hevel did not pay the least attention when a prophetic voice informed him of one of the most important lessons of the Torah. In the above pasuk G-d tells Hevel that he can put the past behind. He can try to succeed. He can change himself and his ways. If he does not try to succeed his Yetzer Harah will immediately encourage him to sin.
He did not listen to the little prophetic voice, indeed, perhaps he was annoyed by it. His response to this prophecy was further rebellion. This is because the sin of murder awaited and welcomed him.
Nechama Leibowitz of Blessed Memory completed this discussion by pointing out that this sin perpetrated by man against his brother was done by the second generation of mankind while the first generation had sinned against G-d. This is the beginning of the history of Humanity.
See Seforno and Nechama Leibowitz, Iyunim Chadashim.