I think we all agree at this point that Trump is an absolutely horrendous human being. Regarding that specific story out of San Francisco, I do think a mistake was made in releasing him. What I'm far more interested in is how he really got the gun registered to a Federal agent. Like in planning, I don't think you can make any kind of blanket statement based on the actions of San Francisco, which has long been regarded as inept when it comes to a variety of things.
It is also worth noting that I have little regard for the opinions of talking heads on immigration. I grew up around immigrants, including a number of illegal immigrants. I get offended pretty easily when it comes to the immigration topic.
The term "Sanctuary City" is little more than a talking point. Yes, there are a few cities that truly fall in that realm, but that list is pretty inaccurate. Including cities with supposedly de facto policies is stupid--police have long had leeway in enforcing laws & ordinances. For example, they don't stop every car going 1 mph over the limit, and they don't ticket every jaywalker. Even that website states that many are not confirmed, using "reader observations." The Congressional Research Service isn't great about documenting either.
I can speak to some degree about Austin, which appears on that list. Their police don't generally ask for immigration status because ICE officials routinely go through their city & county jails. Their leadership wants the cops out on the streets preventing & addressing active crime and building relationships in the community so that crime is reported when it occurs, not duplicating efforts of a Federal agency tasked with enforcing immigration. To put this in planner world, the police typically don't bust somebody for posting illegal signs even though it is against a local ordinance because there are code enforcement officers to deal with that. In Austin, the level to which they reach out to ICE depends on the initial crime--they won't mess with a speeding ticket, but a drug offense with intent to distribute will get you on an ICE processing bus bound for the border.
But let's really get to the heart of this issue, which is whether illegal immigrants have higher rates of criminal activity:
2014 Justice Quarterly Academic Journal says nope; one example of many out there
Over the last two decades, the immigrant population of the U.S. nearly doubled as a share of the U.S. population and is currently around 13% of the total population. During that same time, we went from about 3.5 million illegal immigrants in the U.S. to nearly 12 million. You can go look up the census data on this yourself. Despite this, the FBI has consistently reported significant reductions in property & violent crime over that same time period (also easily found). The decline in crime rates was not just national, but also occurred in border cities and other cities with large immigrant populations.
Even
this report by the conservative
America's Majority Foundation drew similar conclusions, finding that crime rates actually declined in high-immigration states. There are a number of other more academic studies by some economists that show similar results.
The problem of crime in the United States is not caused or even aggravated by immigrants, regardless of their legal status. Immigrants come to the United States to pursue economic and educational opportunities not available in their home countries and to build better lives for themselves and their families; they have little to gain and much to lose by breaking the law. Undocumented immigrants in particular have even more reason to not commit crimes due to nearly guaranteed deportation.
I watched a friend of mine go through the immigration process from Mexico after he overstayed his visa--everyone always thinks of people crossing rivers, but the most common form of illegal immigration is simply overstaying a work visa. It was an insanely broken system for someone that was not any kind of threat and had gainful employment, paid taxes, etc. If you'd like a detailed account of his experience I'll be happy to provide it, but all I will say now is that it was the most frightening experience of his life as he sought his full green card in Juarez.
I'm not advocating open borders by any means, but a clear & easy path to legal status is essential. Immigrants are a key component of this country's economic strength, which many tend to forget.