Rashi explains that Yosef calms his brothers' concerns by telling them
וידבר על לבם. דְּבָרִים הַמִּתְקַבְּלִים עַל הַלֵּב – עַד שֶׁלֹּא יְרַדְתֶּם לְכָאן הָיוּ מְרַנְּנִים עָלַי שֶׁאֲנִי עֶבֶד, עַל יְדֵיכֶם נוֹדַע שֶׁאֲנִי בֶן חֹרִין, וְאִם אֲנִי הוֹרֵג אֶתְכֶם, מָה הַבְּרִיוֹת אוֹמְרוֹת? כַּת שֶׁל בַּחוּרִים רָאָה וְנִשְׁתַּבֵּחַ בָּהֶם וְאָמַר אַחַי הֵם, וּלְבַסּוֹף הָרַג אוֹתָם; יֵשׁ לְךָ אָח שֶׁהוֹרֵג אֶת אֶחָיו? He spoke words that found ready entrance into their heart saying, "Before you came down hither people spread rumours about me that I was born a slave; through you it became public that I am a free-man by birth. If I were to kill you what would people say? 'He saw a party of fine young men and he prided himself on his relationship with them, saying These are my brothers, but afterwards he killed them. Have you ever heard of a man killing his brothers?!”
How did the Egyptians not believe a someone could kill their brother if stories like Kayin killing Hevel, Esav wanting to kill Yaakov, and the like were part of world history?