How did times of social repression bring out the worst charcteristics in humans in The Crucible?
The Crucible is an allegory for any society which punishes those who question the authority of the church, state or any powerful institution. Having been written in 1953, many see the play as an allusion to McCarthyism, during with Senator Joe McCarthy led the House Committee on Un-American Activities on a "witch hunt" to find Americans with communist ties. America is famous for its first amendment, the right to free speech, which implies the right to free thought, so McCarthyism was completely contrary to that right.
As for the content of the play itself, one argument is that the girls who danced naked in the woods acted out because of how repressive (particularly sexual repression) their society was. In fact, the religious fanaticism of Salem, in this play, is so strict that the town becomes hysterical with witch fever and it is all based solely on the accusations of scared young girls afraid of getting in trouble. There is no hard evidence. Therefore, those in power were even repressing themselves (consciously or unconsciously) to the point where they could no longer see or acknowledge what they were doing was wrong.
In fact, Deputy Governor Danforth realizes at a certain point that these accusations could be false, but he thinks it would reflect badly on the court if he pardoned those who were left after some had already been executed. He is more concerned with appearing correct than actually doing the right thing. He and others in this play ignore their better judgment because they don't want to look bad in front of the others; this fear stems from that social (in this case, strict religious and political) oppression.