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.