In The Stranger, is Meursault a threat to society?
Meursault is indeed a threat to society—not just to French society, but to any society. The main danger that he poses is that he doesn't believe in anything. As such, there's little or nothing that society can do to control him. No positive inducement to conform to society's norms and expectations will ever work on someone so thoroughly nihilistic in his moral outlook. For someone like Meursault, killing someone is like crossing the road or putting on a pair of pants. So long as such people are at liberty, there's simply no telling what they might do.
In fact, Meursault's amorality is considerably more dangerous to society than the immorality of the violent pimp Raymond Sintes. Sintes's actions, though utterly despicable, can at least be rationally explained, and as such, dealt with. But there's no possible explanation for Meursault's killing of the Arab, not even self-defense. Attempts at providing some sort of psychological explanation for his actions simply raise more questions than answers. Meursault's total indifference to the death of his mother strengthens not so much the legal case, as the moral case against him. And there can be little doubt that the language used by the prosecutor indicates that Meursault's amorality is also on trial.
In any case, Meursault knows that he can't use mental disorder as a mitigating factor in his defense. He knew exactly what he was doing. In killing the Arab, he made a clear existential choice. In what he believes to be a godless, meaningless universe, he took upon himself the decision to act, to take his own existence in his hands and live authentically. That being the case, he represents a clear and present threat to any society, as societies must be based on rules, standards and moral values if they're to survive. But if we have people like Meursault going around making up their own rules, then the result is likely to be widespread chaos and disorder.
As Camus' Absurd hero, Meursault effectively rebels from society's expectations. He does so, not out of pointless defiance, but out of a principle of genuine behavior. Meursault is a threat to society because has proved himself to be capable of murder; plain and simple. The more complicated implication is that, as Meursault is tried, convicted, and sentenced, it becomes more and more clear that the court (and those of his social world) are convicting him for his lack of religious belief and lack of adherence to their (society's) code of behavior. In other words, he is a threat because he killed a man. But he is tried as much for his philosophical perspective as he is for his actual crime. This is the absurd irony of his situation. The court and the chaplain are more horrified by his beliefs (or lack thereof) than they are by the actual crime he committed.
For example, the Prosecutor, in attempts to establish Meursault as having no remorse, says that Meursault is "morally guilty of his mother's death" because he did not properly mourn. The lawyer compares Meursault's crime to the crime of the upcoming trial: a son arrested for killing his father. The Prosecutor is correct in determining that Meursault is a threat to society because Meursault offers no logical explanation for why he killed the man. But again, the irony is that the Prosecutor is equally, or more, appalled at Meursault's general behavior. The Prosecutor, continuing to focus on Meursault's behavior, goes so far as to say that Meursault "is also guilty of the murder to be tried tomorrow in this court." Meursault is a threat because he killed, but the Prosecutor is condemning him for being a social outcast rather than for being a social threat.
Merusault is a threat to society because he is capable of killing a man but having no reason for doing so. He claims to be a rational man in an irrational word. Meursault is everything that society is not. He doesn't mourn for his mother; he doesn't show any emotion and that frightens people.