Proponents of Arizona's SB 1070 are operating under a number of misleading assumptions and/or deliberate distortions. The undocumented population of Arizona is decreasing, not increasing, drug violence from Mexico has not spilled over, crime is generally down, and deportations are higher than they've been in 10 years.
Proponents of the law know that public opinion is on their side, however, so instead of engaging reality, they've chosen to make outlandish claims that are out of line with the facts but in sync with people's emotions, with Gov. Jan Brewer telling false tales of beheadings in the desert and Sen. Jon Kyl claiming the Obama administration is holding the border "hostage" even as it deploys troops to guard it.
This weekend on Face the Nation, Kyl dismissed the notion that crime was down in Arizona as a "gross generalization":
SMITH: One of the things that’s come to light over the past last couple weeks is that in some of these border towns that were thought to be susceptible to lawbreaking of illegal immigrants. Crime is actually down. Crime in Phoenix for instance is down significantly over the past couple of years.
KYL: Well, that’s a gross generalization. Property crimes are up, certain violent crimes on certain parts of the citizenry are up. Phoenix is a very large source of kidnapping. It’s called the kidnapping capital of the United States. … So there’s a great deal of violence and crime associated with illegal immigrants.
Andrew Nill at Thinkprogress is giving Kyl a hard time for this, but Kyl's right. The idea that crime is down in Arizona is a "gross generalization" in the sense that the one place where it isn't is in the jurisdiction of conservative celebrity Sheriff Joe Arpaio, whose draconian anti-immigrant policies have made him so popular among the nativist right:

That's probably not what Kyl meant.
Between the statements of other law-enforcement officials that having local police enforcing immigration policy would be detrimental to fighting crime and studies showing that increases in immigration correlate with decreases in "homicide and robbery," I think it might be fair to say that the choice here is between making a monumental effort to purge Arizona's undocumented immigrants and the people most likely to be mistaken for them and actually making the state a safer place to live.