Torture is proven to cause deep feelings of hatred and a need for revenge.
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This is an excerpt from a paper I just turned in about Al Qaeda's formation, financial history, why they hate the U.S., etc. The main reference was a book titled "The Looming Tower"
"III.Why is the U.S. a Target of Al Qaeda?
Two early schools of thought that fueled AQ’s desire to target America for terrorist attacks are discussed below.
A.Crimes Against Muslims and Islam
One theory that America was targeted by Al Qaeda on 9/11 begins with the torture of Islamists in the prisons of Egypt (Wright, 61). Sayyid Qutb and his acolytes, including Ayman al-Zawahiri, purportedly acquired a deep desire for revenge after their torture (Wright, 60). Although the primary targets of Islamists wrath was Egypt’s secular government, some of the anger was channeled toward the West as it was seen as an enabler of the regime that committed the torture (Wright, 60). Torture of the Islamists in Egyptian prisons was thus transmogrified into the humiliation of all Islamic society, especially to young radicals (Wright, 60). Revenge against the West was no longer retribution, it was justice (Wright, 60)."
REFERENCE:
Wright, Lawrence. (2006). The Looming Tower - Al Qaeda and the Road to 9/11. New York: Vintage Books – A Division of Random House, Inc.