Why did the Athenians sentence Socrates to death?

Rather than conforming to society, Socrates voiced his rather unpopular opinions to the public, some of which were heretical or deemed obscene by the Athenian government. Socrates had praised the Spartans-Athens' arch-rival-on numerous occasions, both directly and indirectly, and publicly criticized the current political, social, and moral atmospheres at the time. Both of these actions were considered dangerous by the politicians, especially since there was social unrest and instability during Socrates' criticisms.

Instead of leaving him be, the Athenian government put him on trial for heresy and treason. Refusing to flee, Socrates awaited his trial. In the end, he was sentenced to death via poison. Socrates then drank a vial of poison and waited until it reached his bloodstream, numbing his body. The poison finally reached his heart, sending him into cardiac arrest and killing him almost immediately.

In short, Socrates was sentenced to death for heresy, treason, and the corruption of youth.

According to Robin Waterfield in Why Socrates Died: Dispelling the Myths, Socrates played as a voluntary scapegoat for the misfortunes Athens was facing.

See the related links section for more information.