Or HaChaim on Numbers 14:15/אור החיים על במדבר י״ד:ט״ו

1 והמתה…כאיש אחד, "and You kill them..all at the same time, etc." Moses revealed here that he did not plead for G'd to forgive the Israelites their sin; he only pleaded that the retribution should be such that it did not result in a desecration of G'd's name in the eyes of the nations. This is why he emphasised the words כאיש אחד, "as one man."

2ואמרו הגוים, "and the Gentile nations will say, etc." Moses did not only refer to the Egyptians but to all the nations who would hear about the fate of the Jewish people. לאמור, to say; this word again means "something elevated," as we explained on 14,7. All the people who had thus far looked upon the G'd of the Hebrews as a supreme deity would now revise their estimate of G'd's capabilities if they heard that this G'd had wiped out His people as one man. Moses added the letter ו at the beginning of the word אמרו in order to indicate that he referred to all the Gentile nations not only to the Egyptians. Another reason for the open-ended word לאמור is that Moses did not know precisely what the Gentiles would say and in what order. When Moses spoke about what the Gentiles would say, i.e. ואמרו, he did not imply that they would be justified in what they would say. He was only concerned that they would say it anyway.