Who two characters have the strongest opposition and conflict between each other in the Crucible? and Is Tituba a slave?If they do, why?

The morality play designed by Arthur Miller lends itself to poising characters at opposite spectrum of ethical actions.  This means that one could pick two characters who represent different ends and they would have a strong level of opposition and conflict between what they believe and how they live their lives.  No better could this be seen between John Proctor and Abigail Williams.  The former turns out to be a pillar of morality, and one who refuses to speak falsehoods even if it could benefit his own existence.  The latter is one who speaks nothing but falsehoods in her own sense of self interest.  Another opposition which bears similarity to the notion of truth and manipulation can be seen in Giles Corey and Thomas Putnam.  The latter seeks to make profit and generate personal wealth through the misery of those who are accused, while the former speaks out against such a practice, and when he, himself, is accused demands "more weight," implying that he is willing to endure more than others in seeking his own conception of truth and justice.

The idea of creating characters on different ends of the ethical spectrum is done with the understanding that the reader/ audience can see how one extreme represents a noble path and the other is less than that.  An implication from this is to suggest that no one is subjected to the "lesser" path of human action for we as observers see the disastrous results of these actions.  In this vein, while Tituba is a Black slave from the Caribbean, we understand that when she is condemned and one of the first to die, it is a major offense and one that must be avoided.  Someone once said, "It's a minor crime when it happens to someone else, but it's a major one when it happens to you."  Indeed, Miller is trying to persuade us to understand that a miscarriage of justice and an institutional framework that does not have the general interests of all in mind is a "major crime" to all of us.  When Tituba is condemned to die, it is a bad sign for everyone in Salem as it foreshadows what is to come.  It is not something that concerns "those people," as much as Miller is trying to say it impacts everyone.