Is the plot of Oedipus Rex by Sopocles ethically or pathetically motivated?
Ethos and pathos are not terms used to describe plots. Plot structures can stem from character or incident. In the case of Oedipus, the plot structure is based on the working out of the curse on the Theban house. That it will end in disaster is inevitable. The audience would have known the plot in advance of viewing the play. The character of Oedipus, some argue, contributes to his downfall, but once he has killed Laius and married Jocasta, events that occur before the play begins, there is really no way out from under the curse. The chorus states:
Therefore, it is necessary to call no man blessed
as we await the final day, until he has reached
the limit of life and suffered nothing grievous
Ethos means character in the specific sense of the intrinsic character of a speaker's persona as a persuasive rhetorical device, and pathos the swaying of the audience's emotions. It is the pathemata (things suffered) that awaken fear and pity in the audience of drama, not ethical and pathetic persuasion.