Why did American Nancy Astor say Jo Stalin was killing millions?

@FreddieUK I think his death-toll, excluding that of the dreadful Nazi invasion, is estimated at around ten million.

Mao Tse-Tung's was even worse: possibly up to thirty million; mainly by famine but also through purges such as his "Cultural Revolution".

It must be very hard to know as I don't suppose the two governments kept accurate records, especially as they were to blame. I would not be surprised if local officials tried to count low in case they were blamed.