My understanding of the Cato institute result is that they did not refer to all people from the seven middle east countries, but only those who came to the US and naturalized as citizens.  Also, they most likely were very selective in what they called a killing because ordinary per capita murder rates cumulative over so many years would likely have produced a significant number of killings.  Never take libber information at face value.  It is typically slippery and slick with weasel words and hidden qualifiers (not to mention outright lies).