Whom did Kayin and Hevel marry? According to Chazal (בראשית רבה כב ז), Kayin and Hevel were both born with twin sisters. However, Hevel had two sisters born with him, which led to a quarrel over who should marry Hevel's extra sister. This was the cause of Kayin killing Hevel.
Building on this, the Shelah continues that Yisro, who was likewise a gilgul of Kayin, did an act of hachna'ah, an act of submission and humility. He gave his daughter Tzipporah to Moshe, and she was none other than the (reincarnation of the) additional twin who had been at the centre of the original quarrel between Kayin and Hevel. Through this act of surrender, Yisro rectified something of Kayin's ancient wound.
But Korach undid that rectification entirely. He renewed the conflict - and so the reckoning was renewed as well. The Shelah cites the Tziuni, who notes the measure-for-measure precision of Korach's death:
In Parshas Bereishis (ד יא), Hashem reprimands Kayin for killing his brother, with the words: וְעַתָּה אָרוּר אָתָּה מִן־הָאֲדָמָה אֲשֶׁר פָּצְתָה אֶת־פִּיהָ לָקַחַת אֶת־דְּמֵי אָחִיךָ מִיָּדֶךָ - And now, cursed be you from the ground, which opened up its mouth to receive your brother's blood from your hand.
Interestingly, the exact same language is used here, when Moshe Rabbeinu speaks about Korach: וְאִם־בְּרִיאָה יִבְרָא ה' וּפָצְתָה הָאֲדָמָה אֶת־פִּיהָ וּבָלְעָה אֹתָם וְאֶת־כָּל־אֲשֶׁר לָהֶם וְיָרְדוּ חַיִּים שְׁאֹלָה וִידַעְתֶּם כִּי נִאֲצוּ הָאֲנָשִׁים הָאֵלֶּה אֶת־ה' - If Hashem will create a [new] creation and the ground will open up its mouth and swallow them along with all of their belongings, and they will go down alive to the grave, then you will know that these men have angered Hashem.
Just as Hevel's blood cried out from the ground, the ground now swallowed his killer.
(של"ה פרשת קרח, תורה אור אות טו)
