A person can't be “illegal”. A person can commit illegal acts, however, making that person a criminal.
For example, you would not refer to a rapist as “illegal”, you would refer to the rapist as a criminal for having committed the illegal act of rape.
It would be more accurate the refer to these people “Criminal Aliens” as they have committed an illegal act which makes them a criminal.
Don't be suckered down that leftist road. Not only does the law, but even the left, uses the term "illegal" to refer to all kinds of people as in "illegal occupant", "illegal trapper", "illegal food vendor", "illegal trader"....
For generations upon generations, American immigration laws called the foreign born aliens AND separated them in 2 classes "legal aliens" and "illegal alien."
This is crucial because unlike the term "criminal alien", the burden of proof was never on the American government to prove they are "illegal", but on the aliens to prove they are "legal" - as it would be for the former.