still soft-pedaling mass murder

What is it with leftists and murderous communist dictators? In a book review of Mao: The Unknown Story, Nicholas Kristof notes:

In that regard, I have reservations about the book's judgments, for my own sense is that Mao, however monstrous, also brought useful changes to China. And at times the authors seem so eager to destroy him that I wonder if they exclude exculpatory evidence

WTF? If Kristof were to review a book about Hitler, would he be concerned about "exculpatory evidence" as well or mention the wonders of the Autobahn? Sheesh!