Why did the lone surviving slave of Laius's party plead with Jocasta to leave Thebes when he saw Oedipus on the throne in Oedipus Rex?

Laius' sole surviving slave (now working as a farmer)
begged to be allowed to leave when he realized who Oedipus was, specifically, Laius'
murderer.  Oedipus in his arrogance has sworn that he will rid the city of Thebes of its
plague by finding the murderer of Laius, not realizing that he is the one who killed
Laius, nor realizing that he is in fact Laius' son, destined at birth to kill his father
and marry his mother Jocasta (which he did).  Jocasta is trying to learn more about her
late husband's death and so interrogates the slave for information, yet the slave fears
Oedipus' anger if he should tell him the truth.  Therefore, he sees leaving Thebes a
safer option than telling him who the murderer is.