answer the question, we must first identify the perpetrators of the crimes.
It is generally assumed, plausibly, that their origin is the Middle East region and that the attacks probably trace back to the Osama Bin Laden network -- a widespread and complex organisation, doubtless inspired by Bin Laden but not necessarily acting under his control.
Let us assume that this is true. Then to answer your question, a sensible person would try to ascertain Bin Laden's views and the sentiments of the large reservoir of supporters he has throughout the region.
About all of this, we have a great deal of information.
Bin Laden has been interviewed extensively over the years by highly reliable Middle East specialists -- notably, the most eminent correspondent in the region, Robert Fisk, London Independent, who has intimate knowledge of the entire region and direct experience over decades.
A Saudi Arabian millionaire, Bin Laden became a militant Islamic leader in the war to drive the Russians out of Afghanistan. He was one of the many religious fundamentalist extremists recruited, armed and financed by the CIA and their allies in Pakistani intelligence to cause maximal harm to the Russians -- quite possibly delaying their withdrawal, many analysts suspect -- though whether he personally happened to have direct contact with the CIA is unclear, and not particularly important.
Not surprisingly, the CIA preferred the most fanatic and cruel fighters they could mobilize. The end result was to "destroy a moderate regime and create a fanatical one, from groups recklessly financed by the Americans," [according to] London Times correspondent Simon Jenkins, also a specialist on the region. These "Afghanis", as they are called -- though many, like Bin Laden, are not from from Afghanistan -- carried out terror operations across the border in Russia, but they terminated these after Russia withdrew. Their war was not against Russia, which they despise, but against the Russian occupation and Russia's crimes against Muslims.
The Afghanis did not terminate their activities however. They joined Bosnian Muslim forces in the Balkan Wars. The U.S. did not object, just as it tolerated Iranian support for them, for complex reasons that we need not pursue here, apart from noting that concern for the grim fate of the Bosnians was not prominent among them.
The Afghanis are also fighting the Russians in Chechnya and, quite possibly, are involved in carrying out terrorist attacks in Moscow and elsewhere in Russian territory.
Bin Laden and his Afghanis turned against the U.S. in 1990 when they [the U.S.] established permanent bases in Saudi Arabia -- from his point of view, a counterpart to the Russian occupation of Afghanistan, but far more significant because of Saudi Arabia's special status as the guardian of the holiest shrines.
Bin Laden is also bitterly opposed to the corrupt and repressive regimes of the region, which he regards as "un-Islamic", including the Saudi Arabian regime -- the most extreme Islamic fundamentalist regime in the world, apart from the Taliban, and a close U.S. ally since its origins.
Like others in the region, he is also outraged by long-standing U.S. support for Israel's brutal military occupation, now in its 35th year -- Washington's decisive diplomatic, military, and economic intervention in support of the killings, the harsh and destructive siege over many years, the daily humiliation to which Palestinians are subjected, the expanding settlements designed to break the occupied territories into Bantustan-like cantons and take control of the resources, the gross violation of the Geneva Conventions, and other actions that are recognised as crimes throughout most of the world -- apart from the U.S. which has prime responsibility for them.
And, like others, he contrasts Washington's dedicated support for these crimes with the decade-long U.S./British assault against the civilian population of Iraq, which has devastated the society and caused hundreds of thousands of deaths, while strengthening Saddam Hussein -- who was a favoured friend and ally of the U.S. and Britain right through his worst atrocities, including the gassing of the Kurds -- as people of the region also remember well, even if Westerners prefer to forget the facts.
These sentiments are widely shared.
The Wall Street Journal, September 14, published a survey of opinions of wealthy and privileged Muslims in the Gulf region -- bankers, professionals, businessmen with close links to the U.S. They expressed much the same views: resentment of the U.S. policies of supporting Israeli crimes and blocking the international consensus on a diplomatic settlement for many years while devastating Iraqi civilian society, supporting harsh and repressive anti-democratic regimes throughout the region, and imposing barriers against economic development by "propping up oppressive regimes."
Among the great majority of people suffering deep poverty and oppression, similar sentiments are far more bitter and are the source of the fury and despair that has led to suicide bombings, as commonly understood by those who are interested in the facts.
The U.S., and much of the West, prefers a more comforting story. To quote the lead analysis in the New York Times, September 16, the perpetrators acted out of "hatred for the values cherished in the West as freedom, tolerance, prosperity, religious pluralism and universal suffrage." U.S. actions are irrelevant, and therefore need not even be mentioned (Serge Schmemann).
This is a convenient picture, and the general stance is not unfamiliar in intellectual history. In fact, it is close to the norm. It happens to be completely at variance with everything we know, but has all the merits of self-adulation and uncritical support for power.
It is also widely recognised that Bin Laden and others like him are praying for "a great assault on Muslim states," which will cause "fanatics to flock to his cause" (Jenkins, and many others). That, too, is familiar.
The escalating cycle of violence is typically welcomed by the harshest and most brutal elements on both sides -- a fact evident enough from the recent history of the Balkans, to cite only one of many cases.