The GAO estimates “criminal aliens” were arrested, convicted and incarcerated for 25,064 homicides. If non-citizens committed them over seven years, the annual rate would be 14.2 per 100,000 non-citizens. If illegal aliens committed them over four years, the annual rate would be 58.0 per 100,000 illegal aliens. Either way you compute, those are high rates.
Or look at the total number of homicides in those years. Per the FBI, there were 67,642 murders in the U.S. from 2005 through 2008, and 115,717 from 2003 through 2009. Per the GAO, criminal aliens committed 25,064 of them. That means they committed 22% to 37% of all murders in the U.S., while being only 3.52% to 8.25% of the population.