Proctor's motivations change over the course of the drama. While at first, he does not want to get involved, he moves from this over the course of the narrative.
Initially, Proctor does not want get involved in what is happening to Salem. When Abby originally tells Proctor how all of the hysteria over witches is manufactured, he smiles in an almost dismissive fashion. When Elizabeth presses him in Act II about how he needs to tell the authorities what he knows, he deflects it. He rationalizes that it does not meet the threshold of evidence and proof. These actions show that while the town devolves into chaos and emotional contagion regarding the fear of witches, Proctor simply wishes to be left alone.
As the charges escalate, Proctor is forced to take action. When Elizabeth and the others are arrested, Proctor's motivations change. He seeks to disprove Abigail's claim, something that is underscored with his presence at the trials in Act III. As Danforth and Hathorne are driven to find more people guilty, Proctor becomes the avenging angel who seeks to prove that what the girls are saying are false. This desire is evident in how he wants Mary Warren to speak the truth. It is also seen when Proctor admits his adultery to the court in the hopes of ending the world of deceit that has descended upon Salem. When Proctor delivers his final speech in Act IV about the importance of his name, he displays his commitment to truth:
Because it is my name! Because I cannot have another in my life! Because I lie and sign myself to lies! Because I am not worth the dust on the feet of them that hang! How may I live without my name? I have given you my soul, leave me my name!"
At this instant, when Elizabeth says he has his "goodness," Proctor's motivation to reveal the truth is on full display.