There is an indefinable turning point when a word or phrase used to describe a group of people changes from everyday language to offensive slur. Illegal immigrant is somewhere right on the cusp of that point if not past it. Using “illegal” as a noun has long passed that point.
In the past the terms oriental, colored, and tranny were everyday words used with no malice to describe asian, black and transexual people. Each reached a point where the word was taken over by people using it in a pejorative way and eventually it crossed that indefinable tipping point where it was no longer acceptable to use by decent people.
Rather than rehash all the arguments about illegal immigrant as being such a term I’ll just repost some links from another thread.