The Talmud emphasizes the gravity of Cain's act by teaching that whoever destroys a single soul is as if he destroyed an entire world, since all of humanity descended from one person (Adam). The erasure of Abel removed not only him but all his potential descendants.
הֱווּ יוֹדְעִים כּוּ׳. אָמַר רַב יְהוּדָה בְּרֵיהּ דְּרַבִּי חִיָּיא: מְלַמֵּד שֶׁעָשָׂה קַיִן בְּהֶבֶל אָחִיו חַבּוּרוֹת חַבּוּרוֹת, פְּצִיעוֹת פְּצִיעוֹת, שֶׁלֹּא הָיָה יוֹדֵעַ מֵהֵיכָן נְשָׁמָה יוֹצְאָה, עַד שֶׁהִגִּיעַ לְצַוָּארוֹ. וְאָמַר רַב יְהוּדָה בְּרֵיהּ דְּרַבִּי חִיָּיא: מִיּוֹם שֶׁפָּתְחָה הָאָרֶץ אֶת פִּיהָ וְקִיבְּלַתּוּ לְדָמוֹ שֶׁל הֶבֶל, שׁוּב לֹא פָּתְחָה, שֶׁנֶּאֱמַר: ״מִכְּנַף הָאָרֶץ זְמִרֹת שָׁמַעְנוּ צְבִי לַצַּדִּיק״ – מִכְּנַף הָאָרֶץ וְלֹא מִפִּי הָאָרֶץ. אֵיתִיבֵיהּ חִזְקִיָּה אָחִיו: ״וַתִּפְתַּח הָאָרֶץ אֶת פִּיהָ״! אֲמַר לֵיהּ: לְרָעָה פָּתְחָה, לְטוֹבָה לֹא פָּתְחָה. וְאָמַר רַב יְהוּדָה בְּרֵיהּ דְּרַבִּי חִיָּיא: גָּלוּת מְכַפֶּרֶת עָוֹן מֶחֱצָה. מֵעִיקָּרָא כְּתִיב: ״וְהָיִיתִי נָע וְנָד״, וּלְבַסּוֹף כְּתִיב: ״וַיֵּשֶׁב בְּאֶרֶץ נוֹד״.
§ The mishna teaches that the court would say: You should know that cases of capital law are not like cases of monetary law, and would reference the murder of Abel by Cain. Rav Yehuda, son of Rabbi Ḥiyya, says: By employing the plural term for blood, “The voice of your brother’s blood [demei] cries out to Me from the ground” (Genesis 4:10), the verse teaches that Cain caused multiple wounds and multiple injuries to his brother Abel. As Cain did not know from where the soul departs, he struck him multiple times. This continued until he came to his neck and struck him there, whereupon Abel died. And Rav Yehuda, son of Rabbi Ḥiyya, says: From the day the earth opened its mouth and received the blood of Abel, its mouth has not opened again, as it is stated: “From the corner of the earth have we heard songs: Glory to the righteous” (Isaiah 24:16): One can infer that the songs are heard “from the corner of the earth,” but not from the mouth of the earth, as the earth never again opened its mouth. Ḥizkiyya, Rav Yehuda’s brother, raised an objection to Rav Yehuda, son of Rabbi Ḥiyya: The verse states concerning Korah and his assembly: “And the earth opened her mouth and swallowed them up, and their households, and all the men that appertained unto Korah, and all their goods” (Numbers 16:32). Rav Yehuda, son of Rabbi Ḥiyya, said to him: It opened again for a deleterious purpose; it did not open again for a constructive purpose. And Rav Yehuda, son of Rabbi Ḥiyya, says: Exile atones for half of a sin. As initially it is written in the verse concerning Cain that he said: “And I shall be a fugitive [na] and a wanderer [vanad ] in the earth” (Genesis 4:14), and ultimately it is written: “And Cain went out from the presence of the Lord, and dwelt in the land of Nod” (Genesis 4:16). Rav Yehuda, son of Rabbi Ḥiyya, equates “Nod” with “nad,” and understands that Cain was given only the punishment of being a wanderer. Exile atoned for half his sin, thereby negating the punishment of being a fugitive.