In my gut I know that immigrants make neighborhoods safer (at least in this country). I also happen to live in and love a county where 46% of everybody is born in another country. That figure always amazes me... and when you consider the kids of immigrant parents, well, there's just not too much else left.
I get kind of patriotic and sentimental when I think of immigrants and America. My mom is an immigrant as were my dad's parents. Immigrants, past and present, are what makes New York City great and what's made America great. Compared to other countries (the Netherlands included), this is something the US does right. Relatively open borders, a laissez-faire attitude toward immigrants after they're here, and a constitutional right to citizenship for anybody born here (a response to white racism after the Civil War) seem to work pretty damn well, Nativist protests notwithstanding.
The latest issue of Homicide Studies is dedicated to immigration and the evidence, at least judging from this abstract and this one, too, if pretty clear. Immigrants do indeed make neighborhoods safer.