When you see words like “legitimate” and “legal” in front of “citizen” this implies the speaker thinks there are other types of citizens. Which makes them unqualified to discuss civics. These people think their opinion matters more than rule of law. They can’t be trusted. There are citizens, natural born citizens that can run for president, green card holders, and a whole dazzling list of other things but there are no illegitimate or illegal citizens.
Furthermore, not everyone who is here without proper authorization has committed a crime to do so. Sometimes, people come here legally and simply stay longer than they are permitted to. They are subject to deportation if they are discovered, but that is a civil matter, not a criminal one.
This is a subtle point that many people miss in the immigration debate. They assume that everyone who is here without authorization is a criminal (and “illegal”), and that is simply false.