In The Crucible, how and why does the reverend Parris’s behaviour toward witches change through the play?

At first, when the reverend discovers that members of his household: his daughter Betty, his niece Abigail and his servant Tituba, are the chief participants in the inappropriate shenanigans in the forest, his paranoia increases dramatically. Rumours of witchcraft soon circulate in the village, and he desperately wishes that Abigail confesses about their acts. He asks her to be truthful about what they did since his position is at risk. If he should be implicated, he would be immediately voted out of office. He, therefore, tells her: 

Now look you, child, your punishment will come in its time. But if you trafficked with spirits in the forest I must know it now, for surely my enemies will, and they will ruin me with it.

The reverend is persistent and demands that Abigail tells the truth, but she insists that they were only doing it for sport. This, however, does not please him for, he believes, his enemies will use the information against him. It is clear, in this early part of the play, that the reverend is fearful. He wants all the suspicions of witchcraft to be dismissed. He also insists that Mr Putnam should not think of witchcraft when he makes reference to it. He later also asks reverend Hale not to look for evidence of witchcraft, but for evidence which should dispel such horrific rumours.

However, once it has been established through Tituba's 'confession' and Abigail and the other girls' accusations that there is enough reason to suspect witchcraft, the reverend dramatically changes his tune. As soon as he realises that he can benefit from the arrests and the ensuing punishment of his enemies, he fully supports the court. He also realises that as long as Tituba, Abigail and Betty, are witnesses in court to implicate others, attention will be diverted from him.  

It is for these reasons that the reverend repeatedly meddles in the court's business. He consistently reminds judge Hathorne about John Proctor's indiscretions, such as that he works on Sundays, for example. He is determined to have Proctor, Giles Corey and Rebecca Nurse punished. His attitude has now clearly changed. Instead of trying to find reasons to prove that the whole idea of witchcraft in Salem is a fallacy, he insists that the so-called evildoers must be punished. His role as spiritual leader and counsellor is quickly forgotten. He abandons the pulpit and becomes a vengeful accuser, instead of appealing to the court for the salvation, spiritual and otherwise of his congregants.

It is truly tragic that the good reverend has lost the focus of his calling. He, instead, presents himself as a deeply embittered and vengeful persona who is intent on sending those he despises and see as a threat, to their doom.