First off, the fact that they are "undocumented" and here illegally means they have a 100% crime rate. They committed a crime when they snuck accross the border without informing the U.S. authorities. 

Also, this article contradicts your statement:
http://www.heritage.org/immigration/commentary/what-the-media-wont-tell-you-about-illegal-immigration-and-criminal-activity

You're also forgetting that every crime committed by an illegal alien is one that would not have occurred if that alien wasn’t in the United States in the first place.