Parris doesn't really want to spare John Proctor's life, but he realizes that he may have no other choice. By the time Proctor's execution came up, the people of Salem were growing very tired of the hangings and allegations. So many people had been accused of witchcraft that homes and farms were being neglected and falling into disrepair. Everyone lived in fear of being accused, and there were rumors of a rebellion. As the instigator in bringing the judges to town, Parris knows that any blame for what has happened will fall on him. He's not well-liked in Salem anyway.
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