לָמָּה֩ יֹאמְר֨וּ מִצְרַ֜יִם לֵאמֹ֗ר בְּרָעָ֤ה הֽוֹצִיאָם֙ לַהֲרֹ֤ג אֹתָם֙ בֶּֽהָרִ֔ים וּ֨לְכַלֹּתָ֔ם מֵעַ֖ל פְּנֵ֣י הָֽאֲדָמָ֑ה שׁ֚וּב מֵחֲר֣וֹן אַפֶּ֔ךָ וְהִנָּחֵ֥ם עַל־הָרָעָ֖ה לְעַמֶּֽךָ׃
어찌하여 애굽 사람으로 이르기를 여호와가 화를 내려 그 백성을 산에서 죽이고 지면에서 진멸하려고 인도하여 내었다 하게 하려 하시나이까 주의 맹렬한 노를 그치시고 뜻을 돌이키사 주의 백성에게 이 화를 내리지 마옵소서
Or HaChaim
למה יאמרו הגוים, "Why should the nations be able to say, etc.?" Moses suggested that it would be a public desecration of G'd's name to wipe out the Jewish people whom G'd had referred to in Exodus 4,22 as "My first-born son Israel" less than six months previously? It would appear in retrospect that G'd had orchestrated the Exodus only in order to kill this people in the desert? Once all the nations which served idols realised that serving the one and only G'd led to disaster they would be confirmed in preferring the various deities which they were in the habit of worshiping. They would conclude that the fact that the G'd of the Israelites had not killed them (the Gentiles) for worshiping idols proved that it was an extremely dangerous thing to convert to Judaism and to acknowledge the G'd in Heaven.
Rashbam
למה יאמרו?, Moses asks that the people be spared for the sake of G’d’s great name if not for their own sake. Why should His name become desecrated?