There are many potential answers to this question, in part, because it is difficult (if not impossible) to pin the blame on just one character. Abigail Williams would likely be a popular answer because she drank the charm to kill Elizabeth Proctor, accuses Elizabeth of witchcraft in order to resume her affair with Elizabeth's husband, and accuses a great many people of witchcraft. She first accuses Tituba, then Sarah Good, Goody Osburn, Bridge Bishop, Goody Sibber, Goody Hawkins, Goody Booth, and Elizabeth Proctor. Eventually she even accuses her one-time friend, Mary Warren. Abigail is selfish, vengeful, and murderous.
However, one might also blame the Putnams. After all, they admit in Act One that they sent their daughter, Ruth, to conjure the spirits of their dead babies with Tituba. Had they not enlisted Tituba's help, the girls might never have gone to the forest with her and been caught by Reverend Parris; Betty would not have panicked and become ill and Abigail would not have been questioned or have accused anyone else.
You might say that Reverend Parris is responsible for the play's events. He allowed himself to be egged on by Mr. Putnam to announce the witchcraft; he called Reverend Hale to town, inspiring more rumors and doubts; and he tried to sway the court via Deputy Governor Danforth, at every turn, prejudicing the magistrates against the people he does not like and insisting that they are corrupt.
One could also claim that Danforth himself is to blame. He represents the law at the colony of Massachusetts and he could have put down these accusations as meritless when he first arrived (or at any time after). However, he chooses to uphold his authority rather than the truth and permits the hanging of people he seems to realize are actually innocent. The trials add to his power.
Some might argue that Mr. Hale is the one responsible because he is so proud of his knowledge and books that he is rendered blind to the possibility of corruption in either the hearts of the accusers or the judges. He does not stand up to the court until it is too late.
In short, there are so many possible candidates that you could go in almost any direction and find someone who could be blamed for the tragic events.