By being an illegal immigrant they have demonstrated a belief that their personal needs outweigh the needs of others by willfully breaking the law and illegally entering another country. In doing so, they have demonstrated a framework for decision making that doesn't prioritize the rule of law above personal desire. Having already broken the law and gotten away with it, they are more likely to break other laws since they haven't had consequences and may even feel rewarded for the first criminal act.
It does not just have to be illegal immigrants. It can be anyone who has led an unsuccessful life and really had no high points either. There are many legal citizens here in the United States who experience these kind of things as well. We should help these people to stop bad things from happening.