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?

Thewho brings news of
the death of Polybus,'s father, tells him that Polybus was not his birth father.  Instead, he
says, many years ago a shepherd gave the baby Oedipus to him, and he took the baby to Polybus
and Merope, the king and queen of Corinth, who raised him as their own.  Oedipus demands that
the shepherd be found and brought before them.  When this shepherd, once a slave belonging to
Laius, arrives, he confirms that he and the messenger used to tend their flocks together when
they were young, and he admits to giving the man a baby many years ago.  When the messenger
tells the shepherd...