What object does Parris see as an intentional threat to his life?
Parris is concerned that there will be riots in Salem, just as there have been in other towns affected by the witch hysteria. When he conducted the public excommunication of John Proctor, very few people from the community came, and this fact, combined with the upright reputations of those people scheduled to hang at dawn in Act IV has made him quite fearful of the community's anger. He also tells Danforth that when he opened the door to leave his house to come to the jail, "a dagger clattered to the ground." This dagger had, apparently, been stuck into the door, and he has interpreted it as a direct and open threat against his life. He cries, "You cannot hang this sort. There is danger for me. I dare not step outside at night!" Thus, in addition to his concerns for the town, Parris feels (probably greater) concern for himself. He is afraid for his life and he realizes that many in the town blame him for the terrible events and deaths that have already taken place.
In Act IV, Parris comes to the court trembling and afraid. Apparently, when he was using the door to his house a dagger fell:
Tonight, when I open my door to leave my house - a dagger clattered to the ground... There is danger for me. I dare not step outside at night.
This quote proves Parris' justified fear of the people. In fact, it causes him to go to a great risk. He pleads with the judges to not hang this "sort" meaning well-respected and highly revered people of the society. Parris believes that this dagger was just a threat, but that the terror intended will further infect him and that someone may indeed take a greater stab at ending his life.