How is Parris to blame in the Salem Witch Trials? How could he be held responsible?Thanks

In a way, you could say that Reverend Parris is the main cause of the witch trials that occur in the play.

It is Parris who finds the girls in the woods.  You can't really blame him for that.  But he must have raised his daughter in a pretty strange way if her reaction to being caught doing something wrong is to faint and stay unconscious (or pretend to be) for a long time.  So you can blame him for that.

So once she's like that, what does he do?  He starts to try to protect his own reputation and he starts talking about how the devil must be out there among the people.

So you can blame Parris for setting a tone for his daughter and the people that will make them hysterical.  And you can blame him for trying to point the finger at other people so as to make sure no one blames him for what is going on.