What, at its core, was the flaw that Kayin carried - the flaw that Korach, in his final incarnation, failed to uproot?
The Zohar Chadash (בראשית לג ע"ב) states it clearly:
"Rebbi Yehudah said: Kayin came with arrogance, and Hevel came with lowliness of spirit - as it is written (תהלים נא יט), זִבְחֵי אֱלֹהִים רוּחַ נִשְׁבָּרָה - The sacrifices of G-d are a broken spirit - and therefore, וַיִּשַׁע ה' אֶל־הֶבֶל וְאֶל־מִנְחָתוֹ וְאֶל־קַיִן וְאֶל־מִנְחָתוֹ לֹא שָׁעָה - Hashem turned toward Hevel and his offering, but toward Kayin and his offering He did not turn."
This gaavah birthed its natural offspring: burning jealousy. The Pirkei D'Rebbi Eliezer (פרק כא) records:
"Rebbi Tzadok says: Great envy and hatred entered Kayin's heart when his brother's offering was accepted and his was not... He said: I will slay my brother Hevel."
The distance between pride and murder is shorter than we might imagine. When a person's arrogance encounters rejection - when the world fails to confirm the image he has built of himself - the wound can curdle into rage and hatred. Kayin's offering was refused, and what should have been an invitation for self-examination instead became a sentence of death upon his brother.
The tradition adds a remarkable coda. The Pirkei D'Rebbi Eliezer records that Kayin offered flaxseed as a korban, while Hevel brought the best of his sheep. Hashem so much despised the offering of Kayin, which was brought with ga'avah, that He decreed the prohibition of shaatnez - the mixing of wool and linen in a single garment. "Let the offerings of Kayin and Hevel never be mingled," He declared. Even at the level of fabric, the separation between arrogance and humility must be preserved.
