Radak on Genesis 4:3
ויהי מקץ ימים, it was some time after Kayin had become a farmer tilling the soil, that he offered a gift to God. The Torah is deliberately vague about how much time had elapsed, writing only מקץ ימים, at the end of a number of years. It is reasonable to assume that the word ימים refers to years rather than days, as it is clear from Leviticus 25,29 that in that verse the word ימים represents the number of days that make up a year. It would seem therefore, that after having worked the soil for a full year Kayin offered a gift to God, part of what he had harvested. He did this in the spot in which his father Adam had offered prayer and had offered a sacrifice of his own as an acknowledgement of his gratitude, his sons presumably having been in attendance. Although the Torah does not spell out the composition of Adam’s sacrifice, the Talmud in Shabbat 28 suggests that it included a one-horned ox, basing itself on Psalms 69,32 משור פר מקרן מפריס, “ox with horn and hooves,” His sons imitated him, each one bringing a gift from what his labour had produced. The offering was meant to be an expression of man’s gratitude to G’d for the success of his endeavours.
מפרי, the Torah is vague about precisely what the offering consisted of, other than saying that it was part of the fruit produced by the earth. We know nothing about the quality or quantity of this offering. Seeing that when describing Hevel’s offering the Torah adds the words: “from the firstborn of his sheep and from their best ones,” it is reasonable to assume that Kayin’s gift, by comparison, was stingy, in fact represented an insult to G’d rather than a grateful acknowledgment of G’d’s part in making the earth produce a crop for him. This is why his offering was not welcome, was rejected. Had Kayin at least brought of the best quality of the produce he had grown, the Torah would have mentioned this to his credit, just as it had done in the case of Hevel’s offering. As a result of the philosophical contrast expressed by these two offerings, i.e. Kayin’s flax and Hevel’s wool, mixing these two fibres in one garment has been outlawed by the Torah for the Jewish people. The lesson for us is that the gifts proffered by a Kayin and the gifts proffered by a Torah-observant Jew are not compatible with one another.
Radak on Genesis 4:5
ואל קין ואל מנחתו לא שעה, but to Kayin and to his offering God did not turn benevolently. His intention had not been good and pure; God does not need to accept gifts proffered reluctantly, as He is not in need of them. How did Kayin find out that God had not looked with favour on his offering? He found out that Hevel’s offering had been accepted, fire descending on it from heaven and consuming it. Nothing of that kind happened to Kayin’s offering. This caused Kayin dismay and anger and his face became downcast. The expression נפלו פניו, describes that someone feels insulted, depressed.
Plaut-Bamberger Commentary
Both Cain and Abel bring sacrifices to God--only Abel's is accepted; the biblical writer offers no explanation for God's choice.
Some comentators maintain that the key to God's preference may be found in the intent of the two worshippers. While Cain brings merely "an offering," Abel brings "the choicest" of his flock. One performs outward motions, the other offers the service of his heart.
A better interpretarion, however, is that God's rejection of Cain's offering is inexplicable in human terms. God acts in accordance with divine wisdom: "I will be gracious to whom I will be gracious" (Exod 33:19). God's reasons are unknown to us. The inexplicability of divine preferment marks Cain as an essentially tragic character; he reacts with blind violence to a rejection that he cannot comprehend.
Samson Rafael Hirsch on Genesis 4:7
The text says of sin that "its urge is toward you" (4:7). This implies that sin wants to be conquered by us; but if we fail to conquer it, sin returns to God and accuses us."
Pirkei Avot 4:1
Who are strong? Those who master their urge.
Mishnah Sanhedrin 4:5
The Hebrew in 4:10- d'mei Achicha, appears to read "bloods," as if it were collective: Abel's unborn decendants also cried out to God.
Avot De Rebbi Natan 31
From this we also learn that one person's life is equal to all of creation.
Midrash Tanhuma
What Cain said to Abel was, "Come and let us divide the world between us." Abel replied, "very well." So Cain said, You take the flocks and I will take the land." And it was agreed between them that the division was final and neither could take what belonged to the other..
Nevertheless, when Abel proceeded to graze the flock, Cain replied, "the land you are grazing on is mine!" Abel replied, "the wool you are wearing is mine!" Cain proceeded to chase Abel until the two grappled. Abel overcame Cain, so that Cain fell underneath him. Cain, aware of how badly it was going for him, began to plead aloud: Abel, my brother, there are only two of us in the world...what are you going to tell our father?
Abel, filled with compassion for his brother, let him go. At once Cain rose up against him and slew him. How did he slay him? Cain took a stone and inflicted many contusions and bruises on Abel's arms and legs, for he did not know what part of the body the soul goes out of, until, when he got to Abel's neck, Abel died.
Malbim
Kayin did not understand God’s words. He didn’t comprehend that the external person, composed of flesh, sinew and bone, is not the true person, but only a beast having the appearance of a person, and that the true person is the inner person who comprehends that which is concealed within, and directs it. He thought that the body and its strength were all there was to a person, and so when he heard from God’s mouth that sin was with him and crouching at his door and eager to cause him to sin, and that the means were in his hands to conquer it, he did not know about whom these words were said and what this sin was. And because there was no one in the world except he and Hevel his brother, he thought that Hevel’s intent and desire was to lead him down into Sheol and that he had the power to subdue him. And this is what is written, “Kayin said to Hevel his brother,” that he said to himself that God’s intention was against Hevel his brother, and therefore, “he struck him and killed him,” in accordance with what God had said to him, that he would subdue him [to Bereshit 4:8].
Jonathan Ben Uzziel
Abel said: "My sacrifice was accepted because my good deeds exceeded yours." Cain answered "There is no justice and there is no judge, and there is no world-to-come and no reward or punishment for the righteous and wicked." About this the brothers quareled. Cain set upon his brother Abel and killed him with a stone.
Elie Wiesel
One more war. The last. They always say that.Let us fight so as to fight no more. Let us kill so as to conquer death. Who knows, perhaps Cain himself aspired to be not just the first murderer but the last as well.
Rabbi Shimon Bar Yochai
When God asks Cain "Where is your brother, Abel? Cain answers " Am I my brother's keeper? You are God. You created me and it is Your task to watch him, not mine. If I ought not to have done what I did, you could have prevented me but you didn't.
In this way, Cain makes God co-responsible for his own actions.