The great Athenian philosopher Sokrates was brought to trial and executed in 399 BC. He was brought up on official charges of "not holding the sasme gods as the polis, introducing strange gods and of corrupting the youth" (Plat. Apol. 24b; Xen. Mem. 1.1.1). There were, however, some unofficial and very political reasons for his execution, including his preaching on and practicing withdrawal from the political life of the city and, according to Xenophon (Mem. 1.2.12), his relationship with Alkibiades and with Kritias, the leader of the Thirty Tyrants.
The one major thing that Sokrates did to secure his role as a social outcast was to distance himself from the political scene in Athens. As I. F. Stone puts it, "The Athenians believed that the citizen was educated and perfected by participating fully in the life and affairs of the city" (The Trial of Socrates [Boston 1988] 98). Sokrates committed a major faux pas in the eyes of the other Athenian citizens by refusing to make a firm stand for his political beliefs.
Even though Sokrates would not publicly and officially voice his political beliefs, he did hold some firm ones. One of these was his way of thinking about the democratic government that was running Athens. Sokrates was very much against the idea of a government ruled by the people. According to Pomeroy et al., Sokrates believed that "most people aren’t terribly thoughtful or analytical, so why should ‘most people,’ that is the majority, make life and death decisions that affect the polis" (Ancient Greece [New York and Oxford 1999] 324). The Sokratic doctrine even says that "the ‘one who knows’ should rule and the rest obey" (Stone 62). This attitude towards democracy, combined with his association with Kritias and other antidemocrates, could easily have made the people of Athens think that Sokrates was "a purveyor of dangerous ideas" (Pomeroy et al 324).
As mentioned earlier, Sokrates was charged with corruption youth. What lay behind this accusation was, at least in part, his relationship with Alkibiades, his student and supposed lover. Alkibiades was a wealthy young man from one of Athens' most illustrious families. After the death of his father he was raised by Perikles. In his youth he gained a reputation as a playboy and as someone who seemed to consider himself above the law. Though still relatively young (born c. 450), he was a charismatic politician and, according to the historian Thucydides, was instrumental in convincing the Athenians to invade Sicily in 415, an expedition for which he was one of the commanders. Before the expedition set sail, numerous sacred images of the god Hermes, patron of travellers, were mutilated, an act which was probably nothing more than a prank played by spoiled upper-class youth, but which was taken by many as a sign of a serious anti-democratic plot. Suspicion fell on Alkibiades, who was nonetheless allowed to sail as co-commander of the expedition. Later, however, he was recalled to stand trial, but fled instead, first to Sparta, and then to the Persian governor Tissaphernes. Back in Athens, after the moderate oligarchy which had taken power in 411 was ousted, the Athenians were persuaded by Alkibiades that he had the inside track to financial aid from Persia because of his friendship with Tissaphernes, and elected him general. Initially successful in the field, Alkibiades returned in triumph to Athens in 407, but things turned sour after the Athenian loss at Notion, the fault of one of his subordinates, and Alkibiades was not re-elected general for 406. At this point he withdrew to his family's fortified estate in Thrace; then in 404, with the fall of Athens to the Spartans and their installation of the Thurty Tyrants to rule the defeated city, Alkibiades took refuge with the Persian governor of Phrygia, but was soon thereafter killed.
One condition of the ouster of the Thirty Tyrants and the restoration of democracy was a rather broad amnesty, but the Athenians were still in a mood to punish the anti-democratic aristocrats who had formed and supported the Thirty, and whom the Athenians blamed for their defeat in the war. Though Alkibiades was already dead, his memory lived on. Alkibiades could not be directly attacked, but the blame for his behavior could be put on the shoulders of his teacher, Sokrates, who was, after all, know for his unsympathetic feelings towards Athenian democracy.
In sum, Sokrates’ execution was due in no small measure to his pointed absence from Athenian politics, the character of his followers, notably Alkibiades, and because the anti-democratic views he represented were a threat to the status quo. With a reputation for opposing democracy and its trapping, blamed for the wild and arrogant, Sokrates did not lead a life that others in Athens found to be constructive. Rather, his ideas and his life could be seen, whether justifiably or not, as endangering everything his Athenian contemporaries believed in.