Who could've guessed?
People are not illegal, to use an adjective here is to make it a property of the person.
If you wanted to focus on the legality of the act, you could use "person who immigrated illegally". Adverbs modify actions.
Words matter. Dehumanization is the first step towards genocide. Be more careful.
"Law enforcement" is what they called the Gestapo as well. Again, words matter, when used to conceal and propagandize, they make the unthinkable thinkable.