דברים י״ט:י״א-י״ג

וכי יהיה איש שנא לרעהו. עַל יְדֵי שִׂנְאָתוֹ הוּא בָא לִידֵי וארב לו, מִכָּאן אָמְרוּ עָבַר אָדָם עַל מִצְוָה קַלָּה, סוֹפוֹ לַעֲבֹר עַל מִצְוָה חֲמוּרָה, לְפִי שֶׁעָבַר עַל לֹא תִשְׂנָא (ויקרא י"ט), סוֹפוֹ לָבֹא לִידֵי שְׁפִיכוּת דָּמִים, לְכָךְ נֶאֱמַר "וְכִי יִהְיֶה אִישׁ שֹׂנֵא לְרֵעֵהוּ וְגוֹ'", שֶׁהָיָה לוֹ לִכְתֹּב וְכִי יָקוּם אִישׁ וְאָרַב לְרֵעֵהוּ וְהִכָּהוּ נָפֶשׁ (ספרי):

If there is a man who hates his neighbor. Because of his hatred, he will eventually stalk him. From here it is derived that if a man violates a minor offense, he will ultimately commit a grave transgression. Because he violated “Do not hate [your brother],” he ultimately committed murder. This is why it says, “If there is a man who hates his neighbor, etc.” Otherwise, it should have written, “If a man rises up, stalks his neighbor, and wounds him mortally.”

לא תחוס עינך. שֶׁלֹּא תֹאמַר הָרִאשׁוֹן כְּבָר נֶהֱרַג, לָמָּה אָנוּ הוֹרְגִים אֶת זֶה וְנִמְצְאוּ שְׁנֵי יִשְׂרְאֵלִים הֲרוּגִים? (שם):

Do not view him with compassion. Do not say, the first one has already been killed. Why kill this one, and have two dead Jews?