"Illegals" and "Broken Windows"
This was made fairly explicit by Minnesota Governor and (for some reason) presidential hopeful Tim Pawlenty, who recently compared immigration enforcement to the "broken windows" police strategy: "It's analogous in some ways to what was happening in New York not long ago. If you allow people to pee on the sidewalks, next they're snatching purses."
What's going on with the term "illegals" is a statement about human nature. According to conservatives on this issue, there are basically two kinds of people -- lawbreakers and law-abiders. Law-abiders are in the habit of respecting any law. Lawbreakers, on the other hand, have a lowered respect for law as a whole. This means that if someone gets away with breaking one law, they'll break others, because only a sort of generalized respect for the law (which they have demonstrated they lack) keeps people in line.
This is the basic thinking behind the "broken windows" police strategy in New York that Pawlenty referenced -- if people see that you can get away with small crimes like breaking windows of abandoned buildings, then that will tip them over into "lawbreaker" status, leading them to commit big crimes too, while cracking down on little crimes will keep them as law-abiders who wouldn't commit any crimes. Pawlenty thinks the same thing applies to immigrants -- if they get away with the violation of unlawful presence in the US, then that shows they are general-purpose "illegals"* who would happily break any other law too.
This is an empirical claim -- people who break immigration laws are just lawbreaking types and thus have a propensity to break other laws too. And it has been tested. Unfortunately for Pawlenty, those tests have come up negative. Research on immigration and crime has consistently shown that immigrants, including undocumented ones, commit crimes at a lower rate than native-born citizens. And crackdowns on undocumented immigrants do not bring down overall crime rates -- in fact, crime in everywhere in Arizona has been on the decline except in Maricopa County, where Sheriff Joe Arpaio has famously made immigration enforcement a central part of his agenda. It turns out that focusing your resources on checking the papers of anybody with a cracked taillight means you don't have time to go after the real criminals.
A better theory, supported by the actual experience of immigrants themselves, is that when a law is sufficiently unjust and sufficiently burdensome to people, those people come to see that particular law as illegitimate. They then feel less compunction about breaking it -- but that lawbreaking doesn't spill over into their approach to laws they find just and not-so-burdensome, like theft and murder laws. Migrating to the US illegally is not something you do for the hell of it or because you have a sense of entitlement about getting what you want. It's a difficult project that requires a lot of planning and willingness to accept risk -- and thus the people who are successful at it are not going to be inclined to blow it by getting in trouble unnecessarily once they're here.
So the term "illegals" is not just dehumanizing and bigoted. It's also based on an empirically false sociological theory.
*I don't know if Pawlenty himself actually uses the term, but the thinking is the same.