By the end of Chapter 10, Tom is in fear for his life because of what he saw in the graveyard. At the end of the chapter, he finds the brass knob and that is the last straw. To understand why, you have to go back to Chapter 7.
In Chapter 7, Tom is "in love" with Becky Thatcher. They end up getting "engaged." At that point, all is great, but Tom screws up by letting slip that he used to be engaged to Amy Lawrence. At that point, Becky starts to hate him. He tries to bribe her with the brass knob because that is his greatest treasure. She knocks it out of his hand but he hopes she'll take it and (by doing that) show she still loves him.
When he sees the knob there, he knows she doesn't love him and that is why it adds to his troubles.