According to Sacks, Dawkins has misunderstood those sections of the Hebrew Bible because he is a "Christian atheist" rather than a "Jewish atheist".
Dawkins, says Sacks, reads the Old Testament in an "adversarial way," and that is "Christian" because Christianity’s New Testament is supposed to have "gone one better" than the Old Testament.
Alexander Waugh has a nice illustration of this in his book God - The Biography:
Randolph didn't come to the shocking conclusion that God's a shit because he was already committed to reading the Old Testament in an anti-semitic way, but because that's the conclusion that any sensible person would draw after reading it at first blush.
In fact, Dawkins's point is hardly new. As the Christian Paul Copan points out, Enlightenment thinkers like Robert Ingersoll were arguing back in the 19th Century that the God of the Old Testament was a cruel and unjust person, and that no one in their right mind could be a Christian as a result.
As Sacks must surely be aware, Christians, just as much as Jews, have strived to show that the Old Testament God is not the monster he might seem to be. They're still at it. Here, for example, is a Christian Apologist attacking Dawkins et al for concluding that the Old Testament God is a moral monster. Here's another. Here's another. Here's another. Here's another.
Clearly, it's not pro-Christian prejudice that leads people to conclude the God of the Old Testament is a moral monster. Rightly or wrongly, it's the Old Testament itself that leads them to draw that conclusion.
Argue, if you wish (and as the above linked posts do), that those atheists who draw the conclusion that the God of the Old Testament is a monster are reading the OT texts in too literal a manner, or are at least unwarranted in drawing that conclusion based on the texts. But the atheist's mistake, if there is one, is clearly not a product of some sort of deeply-ingrained, anti-semitic culture.