In Act 4, why has Parris fallen apart?

Parris has fallen apart in Act IV for a variety of
reasons.  The first would be that the witch trials, something upon which he had pinned a
great deal of hope, have become the source of anger amongst the people.  Citizens of the
nearby town of Andover have openly rejected the trials in their town and held those who
instigated them responsible.  Parris, always insecure of his own status, is fearful that
such rebellion will come upon he and those who initiated the Salem trials of witches. At
the same time, Parris is mindful of how it will look to execute people such as Proctor
and Rebecca Nurse.  Parris recognizes the respect they command, and given the fact that
he will be standing over their deaths, he rightly recognizes that he will receive a
heaping of negative publicity over this.  It was one thing to execute individuals like
Sarah Good or Tituba, people who lacked social power and were marginalized in their own
right.  Yet, Parris understands that killing socially accepted individuals will not
yield good results for him.  On a personal level, his insecurity has been turned up by
Abigail stealing his money and running away, lending even more doubt to the proceedings
as the star witness has run off.  Parris also shares that he has been the victim of
death threats, along with a dagger in his office.  It is for these reasons that Parris
comes across as completely fragmented and incoherent, to a great
extent.