Interesting question. As for Excalibur's response being Mohammed, the number of people he killed will never be completely known, but the killings in the name of his religion make the others pale in comparison.
One thing that can be a factor in man's inhumanity to man is ideology.
Nazi Party: National Socialist German Workers Party.
Soviet Union: Marxist/Socialist political ideology.
China under Mao Zedong. Marxist/Socialist ideology.
North Korea: Marxist/Socialist ideology.
Laos: Pol Pot. Marxist/Socialist ideology.
Islam: Theocracy. Specifically Islamic ideology.
That's not to let other types of governments off the hook.
United States: Approximately 12 million indigenous people were killed in the forming of the nation.
If we look at DNA, the Bonobo Chimpanzee is closest to our DNA and is a vicious tribal ape and kills outside interloping Chimpanzees. We come by our behavior naturally. We are, at the very basic level, tribal in nature.
Theocratic and Marxist/Socialist ideologies contribute to an us versus them mentality, especially with the Abrahamic religions.