“When the debate is lost, slander becomes the tool of the loser.”
According to Plato's Apology, Socrates's life as the "gadfly" of Athens began when his friend Chaerephon asked the oracle at Delphi if anyone were wiser than Socrates; the Oracle responded that no-one was wiser.
Socrates believed the Oracle's response was not correct, because he believed he possessed no wisdom whatsoever.
He proceeded to test the riddle by approaching men considered wise by the people of Athens—statesmen, poets, and artisans—in order to refute the Oracle's pronouncement.
Questioning them, however, Socrates concluded: while each man thought he knew a great deal and was wise, in fact they knew very little and were not wise at all.
Socrates realized the Oracle was correct; while so-called wise men thought themselves wise and yet were not, he himself knew he was not wise at all, which, paradoxically, made him the wiser one since he was the only person aware of his own ignorance.
Socrates's paradoxical wisdom made the prominent Athenians he publicly questioned look foolish, turning them against him and leading to accusations of wrongdoing.
Socrates defended his role as a gadfly until the end: at his trial [Socrates was eventually executed for being a little too much of a freethinker], when Socrates was asked to propose his own punishment, he suggested a wage paid by the government and free dinners for the rest of his life instead, to finance the time he spent as Athens' benefactor.