Islam is, beyond question, one of the most violent religions in human history. What makes it special is that the founder, Muhammad, directly ordered his followers to kill all non-believers. He also ordered them to make no changes whatsoever in Islam as he founded it.
Only the Aztec religion was bloodier.
The wars of Muslim expansion were the bloodiest in history until that point. The conquest of India, alone, killed some 80 million persons. Don't believe it? Look it up. The Muslim expansion made the Inquisition and the Crusades look like Girl Scout picnics in comparison.
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http://www.americanthinker.com/articles/2014/05/the_greatest_murder_machine_in_history.html
The Islamic conquest of India is probably the bloodiest story in history. -- Will Durant, as quoted on Daniel Pipes site.
According to some calculations, the Indian (subcontinent) population decreased by 80 million between 1000 (conquest of Afghanistan) and 1525 (end of Delhi Sultanate). -- Koenrad Elst as quoted on Daniel Pipes site
Over 110 Million Blacks were killed by Islam.
... a minumum of 28 Million African were enslaved in the Muslim Middle East. Since, at least, 80 percent of those captured by Muslim slave traders were calculated to have died before reaching the slave market, it is believed that the death toll from 1400 years of Arab and Muslim slave raids into Africa could have been as high as 112 Millions. When added to the number of those sold in the slave markets, the total number of African victims of the trans-Saharan and East African slave trade could be significantly higher than 140 Million people. -- John Allembillah Azumah, author of The Legacy of Arab-Islam in Africa: A Quest for Inter-religious Dialogue
Add just those two numbers alone together, and Islam has surpassed the victims of 20th-century totalitarianism. However, it does not end there. Add the millions who died at the hand of Muslims in the Sudan in our lifetime.
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Islam has exceeded both Nazism and Marxism in the mass murder sweepstakes. So it is not religious prejudice, but historical fact, that powers the conception of Muslim violence. Islam moderated during the "Golden Age", but has largely sunk back into abject barbarism since WW2.
Today's ISIS is following exactly the route of Muhammad, beheading, stoning, burning, etc. ISIS' latest escapade can be found here:
http://www.el-balad.com/1685658
Nice bunch of guys, eh?