If this question is regarding Meursault's killing of the Arab man on the beach, then yes, he is undoubtedly guilty. Even Meursault admits as much. He went onto the beach, was bothered by the heat and glare from the sun, saw the Arab holding a knife, and then shot the man dead, continuing to fire into his prone body even after it was clear that he was no longer responsive.
Beyond the murder, Meursault is accused of something else: being a monster. His lack of grief at his mother's funeral appears to horrify the courtroom more than his committing murder. Even during his interrogation by his lawyer and the magistrate, Meursault's lack of emotion regarding his mother's death and his lack of explanation for killing the Arab disgusts the other men.
When the magistrate thrusts his crucifix into Meursault's face (evoking vampire stories such as Dracula, with Meursault being the evil monster in this case), he demands that he believe in a moral order to the universe.
But, before I could get the words out, he had drawn himself up to his full height and was asking me very earnestly if I believed in God. When I said, "No," he plumped down into his chair indignantly.
That was unthinkable, he said; all men believe in God, even those who reject Him. Of this he was absolutely sure; if ever he came to doubt it, his life would lose all meaning. "Do you wish," he asked indignantly, "my life to have no meaning?" Really I couldn't see how my wishes came into it, and I told him as much.
By going as far as to accuse Meursault of trying to strip other peoples' lives of meaning by rejecting Christianity and other types of meaning-based views of the world as well, the magistrate is doing more than suggesting that Meursault is a bad man. He is suggesting that he is a moral poison to society because his absurdist vision of life might cause others to doubt that life has meaning.
Is Meursault guilty of being a monster? Is he a danger to society? That depends on the reader, of course, though Camus does not appear to believe that Meursault is anything but an honest man who refuses to pretend to be something he is not.