The story of Cain and Abel is one of the most famous stories in The Bible. The conflict between the brothers, prior to the sin, is not revealed to us in the text. We have to guess what caused Cain to kill his brother, with the help of the verses before the murder.
The story starts optimistically; two brothers were born to Adam and Eve, as we read in Genesis 4:1-2-
"וְהָאָדָם, יָדַע אֶת-חַוָּה אִשְׁתּוֹ;
"And the man knew Eve his wife;
However concise those two verses may seem, they actually reflect the beginning of the tension between the brothers. Cain was born first and Eve explained his name in relation to The Lord; she created a man with the help of G-d. Even though he was still a baby, Cain was considered "a man", in the eyes of his mother. This reminds me of the expectations that parents have of their children. Here, maybe Eve expected Cain to mature quickly and become a man, with the help of The Lord, as soon as possible!
Perhaps if Adam had named him, his destiny would have been different. In any case, Eve's actions seemed to have produced the opposite of what she had intended.
Another point of view has to do with the relationship between Adam and Abel. After his sin in the Garden of Eden, Adam was able to distinguish between good and evil. He probably also understood that he wouldn't live forever and that it was time to create descendants. Maybe he just slept with Eve, without paying attention to the tension that was created after the birth of the 2 sons. A midrash called "Pirke De-Rabbi Eliezer" tells us another story about the "knowing" of the man. According to this midrash, Adam knew that Eve was pregnant from Satan and not from him.
Abel, on the other hand, was born second, with a negative connotation to his name (foolishness, useless, etc.). Yet his occupation appeared first. Does this mean that he was more important, due to his profession as a shepherd? We know from The Scriptures that the shepherd is a sign of a leader, like Moses the prophet or King David. We also know from other mythologies from the area that there were fights between the workers of the ground and the herders of the flocks. Is this story revealing to us the nature of this tension and those conflicts?
The Jewish sages noticed within the above mentioned verses, one word that was repeated three times -the word 'et', which means 'with'. From this repetition they interpreted, that Cain had a twin (girl) that was born with him and Abel had two more girls, who were born with him. Were the two brothers fighting over those girls? (According to some of the Jewish sages, such as Rashi, and also Muslim interpretations, the name of the brothers, who fought over the twins were Kabil and Habil).
The once implied conflict between the brothers now continues conspicuously, as we read in Genesis 4:3-5-
וַיְהִי, מִקֵּץ יָמִים;
"And in process of time it came to pass, that Cain brought of the fruit of the ground an offering unto the LORD. And Abel, he also brought of the firstlings of his flock and of the fat thereof. And the LORD had respect unto Abel and to his offering; but unto Cain and to his offering He had not respect. And Cain was very wroth, and his countenance fell."
Cain's offering wasn't as good as Abel's, because he gave fruit from the ground and that's it, whereas Abel gave the best meat from his flock, including the fat portions. So The Lord favored Abel's offering over Cain's and therefore he reacted to Abel's offering, before reacting to Cain's.
In the Epistle to the Hebrews 11:4, we read:
בָּאֱמוּנָה הִקְרִיב הֶבֶל לֵאלֹהִים זֶבַח טוֹב מִקָּיִן אֲשֶׁר הָיָה־לוֹ לְעֵדוּת "
"כִּי צַדִּיק הוּא בְּהָעִיד אֱלֹהִים עַל־מִנְחֹתָיו וּבָהּ עוֹדֶנּוּ מְדַבֵּר אַחֲרֵי מוֹתוֹ
"By faith Abel offered to God a more acceptable sacrifice than Cain’s. Through this he received approval as righteous, God himself giving approval to his gifts; he died, but through his faith he still speaks".
Like Abel's faith, the story still speaks and will speak forever!