Oedipus assumes there's a plot against him, because nothing else fits the facts (unless Tiresias' allegations are true ...). And remember, as far as Oedipus knows, the only "evidence" for Laius' murder as a cause for the plague came from Creon. He believes Creon has sent Tiresias to try to frame him, in order to take over as ruler. Why should Oedipus believe that Tiresias is a bogus prophet?

• He said nothing about the murder at the time
• He failed to solve the riddle of the Sphinx
• He's a blind old fool who's never said anything useful