Who is the cause of Proctor's tragedy and why?

One could argue that Abigail Williams is to blame for John Proctor's tragedy.  It is she who accuses his wife, Elizabeth, an action that prompts him to bring Mary Warren to the court to tell the truth, that Abigail and the other girls are lying, in an attempt to save his wife.  It is this action that leads the girls to accuse Mary, and then she turns on Proctor and accuses him.  This accusation leads, eventually, to his execution.  Thus, with her accusation of Elizabeth Proctor, Abigail set into motion the chain of events that ultimately led to John Proctor's death.

However, one could also argue that John, himself, is to blame.  If he hadn't cheated on his wife with Abigail, then Abigail would have no reason to believe that he would choose to be with her if his wife were out of the way.  One might say that it was his infidelity that prompted Abigail to accuse his wife and set into motion the events described above.