It’s a truth universally acknowledged amongst those inclined towards new atheism that Karen Armstrong… how shall I put this, has a tendency to sugar-coat the more problematic aspects of religious belief and practice.
Here’s a chunk of stuff I wrote, which didn’t make it into the final version of Chapter 2 of Does God Hate Women?, that shows this up in the way that Armstrong deals with Muhammad’s treatment of the Jewish Qurayzah tribe.
The feeling that things are too good to be true is a frequent experience when reading Armstrong’s writing on Islam. Not least, she seems determined to explain away anything that might show Muhammad in a bad light. For example, about his conflict with the Jewish tribes of Medina, which culminated in the summary execution of 700 males of the Qurayzah tribe, she says variously:
Muhammad had been greatly excited by the prospect of working closely with the Jewish tribes…His disappointment, when the Jews of Medina refused to accept him as an authentic prophet, was one of the greatest of his life. 
In Medina, the chief casualties of this Muslim success were the three Jewish tribes of Qaynuqah, Nadir and Qurayzah, who were determined to destroy Muhammad…They had powerful armies, and obviously posed a threat to the Muslims. 
The massacre of Qurayzah was a horrible incident, but it would be a mistake to judge it by the standards of our own time. This was a very primitive society… an Arab chief was not expected to show mercy to traitors like Qurayzah. 
Muhammad’s intransigence towards Qurayzah had been designed to bring hostilities to an end as soon as possible… Arabia was a chronically violent society, and the ummah had to fight its way to peace. Major social change of the type that Muhammad was attempting in the peninsula is rarely achieved without bloodshed. 
The struggle did not indicate any hostility towards Jews in general, but only towards the three rebel tribes. The Quran continued to revere Jewish prophets and to urge Muslims to respect the People of the Book…. Anti-semitism is a Christian vice. Hatred of the Jews became marked in the Muslim world only after the creation of the state of Israel in 1948… 
In other words, Armstrong’s argument here is that Muhammad really wanted to be friendly with his Jewish neighbours, but they were out to get him, so he exiled and massacred them, but that was okay because these were primitive times, it was necessary, and anyway this kind of thing was not indicative of hostility towards Jews in general, since that was a Christian invention. This sounds like it must be a parody of Armstrong’s views, but actually it seems not to be. Consider, for example, how she describes the events that led Muhammad to banish the Nadir tribe from Medina:
Muhammad tried to reassure Nadir, and made a special treaty with them, but when he discovered that they had been plotting to assassinate him they too were sent into exile… 
Muhammad’s behaviour does not seem particularly objectionable here: indeed, he might have been expected to deal more harshly with the Nadir than simply expelling them; after all, they had been plotting to kill him. Except here is the real story of how he discovered the plot, as related by Martin Lings in his acclaimed biography of the Prophet:
While they were sitting there, in front of one of the fortresses, Gabriel came to the Prophet, unseen by any save him, and told him that the Jews were planning to kill him and that he must return to Medina at once. 
In other words, Muhammad did not discover a plot at all: it was ‘revealed’ to him by the Angel Gabriel in a vision that only he saw. As Armstrong must surely realize, this will hardly do as a justification for expelling an entire community from their homes. Certainly one recalls the scorn meted out to George W. Bush when he was reported as claiming that God had directed him to liberate Iraq.  But it seems that Muhammad is to be held to a different standard. 
 Karen Armstrong, Islam: A Short History, p. 14.
 Ibid, pp. 17-8.
 Ibid, p. 18.
 Ibid, p. 19.
 Ibid, p. 18.
 Peter Lings, Muhammad: His Life Based on the Earliest Sources, p. 209.
 See, for example, The Guardian, October 7, 2005, retrieved June 18, 2008 from http://www.guardian.co.uk/world/2005/oct/07/iraq.usa.
 To be fair to Armstrong, in her first biography of Muhammad she does mention that the Angel Gabriel apparently played a role in these events. But she cannot resist adding the caveat that “a divine revelation would not have been strictly necessary… Muslim sources claim to know exactly who was about to drop a boulder on to Muhammad from a nearby roof-top.” (See Karen Armstrong, Muhammad: A Biography of the Prophet, pp. 193-4).