The illegal immigrants cause mental anguish to people who have been trained for decades by dogma and propaganda to hate immigrants.
In the 90's it was "THEY'RE TAKING OUR JOBS", in the 2000's it expanded to many more things like "THEY'RE GETTING HEALTHCARE FOR FREE" and "THEY'RE CAUSING A TON OF CRIME!" Both of which we all know are false, but to someone who is constantly exposed to right wing media and right wing social spheres, they've been indoctrinated with the idea that Mexicans = bad. So they twist themselves into knots trying to block them out of the country.