According to I. F. Stone’s Trial of Socrates, the real reason Socrates was executed in 399 B.C.E. was that the people of Athens blamed him for spreading anti-democratic teachings. Not only did he spread them, but two of his pupils, Critias and Alcibiades, were active in serious attempts (411 B.C.E. and 404 B.C.E.) to overthrow Athenian democracy. In the eight months that Critias and the Thirty Tyrants actually ruled, they persecuted democrats, executing some 3% of the population of Athens and expelling another 10%. Stone argues, with support from writers in later antiquity, that the restored democratic government of Athens blamed Socrates for the training of Critias, and sentenced him to death chiefly for this reason.

Karl Popper, on the other hand, recognizes the deeply authoritarian trend in our records of Socrates, but blames this on Plato. He feels that Plato misrepresents the historical Socrates, who held what he calls equalitarian and anti-authoritarian views. (Popper’s arguments are presented in his magnificent essay, The Open Society and its Enemies).

You can’t help noticing that Socrates’ reputation is awfully high in antiquity, and since. Classical writers such as Epictetus and Plutarch use him constantly as an example of the ideal philosopher, embodying ataraxia at all times (hence the stories of his equanimity in the face of his wife’s irrational rage). Popper too seems to hold Socrates as an ideal philosopher. But I am persuaded by Stone’s case. I wonder if Socrates’ reputation is not in fact the result of a PR-job by his students.

Anacreon comments:
I think Stone's dead wrong. Had the Athenians really executed Socrates for supporting Tyranny (and there is evidence even in Plato's writings that he didn't) they would have fractured the entire scheme of post-war Athenian society. The only reason a civil war was prevented in Athens after the democratic revolution/restoration was because of the amnesty given by the new regime to all those involved in the revolutionary aristocratic governments that preceded it. If then they would have started to hunt down anti-democrats (even under different pretexts) it would have seriously endangered social stability in a way the Athenians could not afford at the time.

There's an excellent article (excellent even if it was published over 30 years ago) on this subject by Alexander Fuchs