What does The Crucible tell us about the nature of people?
The Crucible was written as an allegory for, and criticism of, the McCarthy-era persecution of those perceived as having communist ties or sympathies. Thus, the play reflects a wide range of personalities and motivations, and while we can say that it reflects human nature, we can't really say that these reflections are uniformly true for each character.
One important element, which is largely responsible for the story taking place at all, is the idea of childhood innocence. In keeping with history (the real Abigail Williams was 11 at the time of her accusations, Betty Parris was 9) the accusers, despite being children, are assumed to be telling the truth. While this might be partly because the people of Salem want to hear their suspicions confirmed (that there are witches in town) it is equally interesting that they presume innocence on the children's part, despite the fact that we all know children are capable of outrageous lies to save them from getting into trouble. Fundamentally, people want to assume that children are innocent. There is probably a form of cognitive dissonance (i.e. reality conflicting with conceptions) at work on the people of Salem, causing them to forget their better judgment because it would be too difficult to face the possibility of children lying about such a serious and deadly subject.
The play also tells us that fear and punishment are powerful motivators. Punishment in particular is sometimes identified as the most fundamental element of morality and social control; things are considered "bad" because they bring punishment, not because of any intrinsic moral transgression. Many times throughout the play we see characters reversing themselves and endangering others in order to avoid punishment.
I'm making a bit of a stretch with my third example and taking a quote from the film "Men In Black" - "A person is smart. People are dumb, panicky dangerous animals and you know it." This perfectly applies to the circumstances of The Crucible: while each individual character evidences intelligence, shrewdness, negotiation and diplomacy, and so forth, when the town is drawn together it degenerates into a mob, seeking only to protect itself. Almost every character is ultimately concerned only with their own well-being, often at the expense of others; this leads them to reject a more rational approach and throw themselves into a black-and-white mindset, such as Danforth prescribes;
You must understand, sir, that a person is either with this court or he must be counted against it, there be no road between.
The reason for this seeming insanity is not that religion is evil, or that the people of Salem have lost their minds. It is simply that they are afraid, and seek answers; answers will not come when the questions themselves are in doubt. This leads to some of the veneer of civilization being stripped away, revealing the monstrous yet terrified animal in us that it tries to hard to subdue.