Or HaChaim on Exodus 32:12/אור החיים על שמות ל״ב:י״ב

1 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.

2The reason Moses added the word לאמור was that the Gentiles might not phrase this in these exact words; nonetheless it was clear that they would draw inferences of that nature when they heard what had befallen the Jewish people in the desert. Alternatively, Moses may have added the word לאמור to show that he knew that this would merely be talk and would not describe the truth; still it was an argument the Gentiles were going to use which it would be difficult to disprove. This misrepresentation of the truth would not be confined to Egypt but would be echoed -לאמור- by all the other nations on earth. There could not be a greater desecration of G'd's name.