In Sophocles' Oedipus the King, the title character finally discovers his true identity after an old Theban shepherd is brought in for interrogation.
As was the case with Oedipus' earlier encounter with Teiresias, the old servant does not want to answer either the questions of Oedipus or those of the Corinthian shepherd. When the old Theban servant is uncooperative, Oedipus threatens him with violence:
If you won’t tell us of your own free will,
once we start to hurt you, you will talk.
(Ian Johnston translation)
Oedipus then tells a couple of his other servants to come over and tie the old man's hands. Moreover, Oedipus then threatens the man with death if he does not answer his questions:
If I have to ask again,
then you will die. (Ian Johnston translation)
Oedipus' threats of violence against the old man strike me as true marks of a tyrant. The Greek title of this play is Oidipous Tyrannos, (Oedipus the Absolute Ruler). Such threats of violence and death, even against an old man, seem to provide further evidence that Oedipus is a true tyrant. He knows he has killed an older man at the place where the three roads meet; earlier in the play he has threatened an old blind man (Teiresias); and now he threatens an old slave with violence and death.
Perhaps we can admire a person who is relentless in his pursuit of the truth, but the means which Oedipus uses to discover this truth appears quite extreme.