I think that Parris' motivation is to consolidate his own control and position as head of the clergy. Parris is shown to be a very insecure man. He is one who cannot accept the premise that he is not beloved, or at the very least, feared. Parris is someone who craves power and spotlight, along with the trappings of each. It is this motivation that drives him throughout the play. As he realizes that the withchhunt deflects criticism as to what his daughter and niece were doing in the woods that night, he recognizes how beneficial the hysteria is to him. As the town Reverend, he would be exalted and his counsel would be sought. Miller depicts Parris as someone who has little care for spirituality, so long as worship brings rewards to his own position of power. It is here where Parris' motivation is most evident for all to see. He simply wants to remain in control for as long as possible and ensure that while so many suffer, he remains in power and at the pinnacle of the social and political domains in Salem. It is this that becomes his motivation until Act IV.