It is hard to assign blame to specific characters because there are so many who are complicit in allowing the witch trials to happen. Many people in Salem (with some exceptions such as Giles Corey, Francis and Rebecca Nurse, and Elizabeth Proctor) get caught up in the hysteria and are therefore contributors to the entire debacle. But there are a few characters who have the authority and responsibility to stop the trials or add to the madness.
For example, Deputy Governor Danforth is shown evidence that those accused might be innocent but he's more concerned with the reputation of the court than he is with justice. He illogically reasons that since witchcraft is an invisible crime, there can be no witnesses to defend the accused.
Therefore, who may possibly be witness to it? The witch and the victim. None other. Now we cannot hope the witch will accuse herself; granted? Therefore, we must rely upon her victims--and they do testify, the children certainly do testify. (Act 3)
Using this logic (or lack of logic), Danforth has essentially made it impossible for those accused to defend themselves.
We can blame the town as a whole for being so religiously fanatical that they are easily terrified by the girls' story. But since Danforth has the most power in these proceedings (and considering he's seen evidence that the accused could be innocent, and does nothing), he's quite responsible. Danforth is in a position of responsibility. He was elected or appointed to make responsible decisions. Therefore, with that position of power comes more responsibility. For similar reasons, Reverend Parris, the local religious authority, also fails to even attempt to stop the trials.
One could also make the case that John Proctor, by taking advantage of Abigail, instigated everything. However, he does try to make things right.
Danforth is in a position of authority and he shows no sense of justice with this responsibility. Other than Abigail, he does the most to exacerbate the hysteria when he orders the arrest of the 91 Salem farmers who sign a deposition stating the good character of Elizabeth, Rebecca, and Martha.