Far as I am concerned, this has nothing to do with Islam. It is carefully directed evil undertaken at the behest of a dictator.
Sure, people who called themselves Christians did vile things during the crusades. The Mongol hordes also. In the Middle Ages, the Moslems were relatively benign, though not without blemish. Up until very recently, the notion of a brotherhood of man was the rare exception.
But when I said there were folks who did worse things than al Qaeda, I was thinking about life today. We are responsible for our own era and not for those which came before us. Excluding Africa, in which anything seems possible, the only comparable act I can think of was the Beslan school massacre.