Are they here legally? No, thus they are illegals. It's not classifying them as "illegal humans", it's classifying their status on US Soil. Get a grip and grow up.
Well, no, the phrase is primarily for dehumanizing them. If your relative got addicted to opioids and did something illegal to get some, would you like people to just call your relative 'the illegal' - not a name, not 'your brother', not even 'the addict', not even referring to them as a person?
Think about what dehumanizing the group of people in question WOULD look like, and ask, does calling them just 'illegals' help or hurt that dehumanization?
I don't even really object to the phrase that strongly in its literal meaning. The issue I have is that dehumanizing effect. We're already incredibly sheltered, unconcerned, arrogant about the conditions of most of the people in the rest of the world - we don't really need to encourage not paying attention to them as people even more.
When's the last time you heard a trumpista begin a sentence with, "I'm concerned about the severe problems people in Central America face; to help them do better I think we should..."
The Republicans view on 'illegals', including refugees, generally goes beyond crazy rage and hate, viewing them as a disease infecting our country and threatening them, who they don't care if they starve, drown, go without medical care, are terrorized, or anything else, only that they STAY OUT,