In Arthur Miller's play "The Crucible," John Proctor's downfall is ultimately caused by his refusal to comply with the demands of the Salem witch trials. Proctor is a farmer who is accused of practicing witchcraft and is subsequently brought before the court.
One of the main causes of Proctor's downfall is his stubbornness and refusal to confess to a crime he did not commit. Despite being offered leniency if he confesses and testifies against others, Proctor refuses to go along with the lies and manipulation of the court. He believes that confessing would mean admitting to a sin he did not commit, and he is not willing to compromise his integrity and reputation in this way.
Another factor in Proctor's downfall is his affair with Abigail Williams, which is used against him by the court as evidence of his guilt. Proctor's relationship with Abigail was seen as a betrayal of his wife, Elizabeth, and was used as a weapon against him in the court proceedings.
Finally, Proctor's downfall is also caused by the corrupt and manipulative nature of the Salem witch trials themselves. The court is willing to do whatever it takes to secure convictions, even if it means fabricating evidence and coercing false confessions. In this context, it becomes nearly impossible for Proctor to clear his name and avoid being found guilty.
Overall, John Proctor's downfall is the result of his refusal to go along with the lies and corruption of the Salem witch trials, as well as the consequences of his affair with Abigail Williams.