Illegal immigrants have been responsible for some horrendous crimes, which is one reason people want to see illegal immigrants deported. There's also the belief that immigrants, legal or not, take jobs away from people born in the country. In fact, however, there's evidence that illegal immigrants are on average actually less likely to commit crimes than people born here and probably help the economy more than they hurt it.
A January 29 report by Gene Demby on National Public Radio points out that so-called "sanctuary cities" -- those where law enforcement agencies have a policy of not enforcing immigration laws -- have a lower crime rate than American cities as a whole and also have stronger economies. That's not necessarily a cause-and-effect relationship, but it makes it hard to argue that providing a haven for illegal immigrants hurts a city. Demby cites an analysis by Tom K Wong at the University of California-San Diego (UCSD) found that cities with a policy of not complying with 48-hour holds initiated by federal Immigrations and Custom Enforcement (ICE) had substantially fewer crimes relative to population, better incomes, lower unemployment, and a smaller percentage of children and teenagers getting public assistance.
Of course, most of us favor enforcement of laws and secure borders, and illegal immigrants are by definition here illegally. But deporting the 11 million or so illegal immigrants living here (down, incidentally, from about 12 million a decade ago before President Obama greatly increased deportation and border enforcement) would be horribly disruptive to the economies of the places where they live. Even Donald Trump, who campaigned on massive deportation, has quietly moderated his stance since taking office and now embraces essentially the same policies put in place by Obama.