In exercising your imagination in trying to create a metaphor of the knife and the shiftless Bob Ewell, you may wish to consider the following:
1. A rusty knife appears worthless, but it can still cause trouble. The citizens of Maycomb perceive the area by the dump as one that no one would consider of any worth; the occupants of this area have no social standing, they have no valuable possessions. However, when the trial of Tom Robinson makes its presence know to all of Maycomb, there are both conflict and stressful moments wrought by Bob and Mayella Ewell.
2.It cannot cut properly, instead tearing at a thing. Bob Ewell appears cocky in court, sure that he will be instrumental in sentencing Tom for the murder; however, he sullies and tears at all that is decent in Maycomb in language that is offensive, as well.
3. Its appearance suggests there will be problems. Just as one knows that the rusty knife will not work as well as a new, sharpened knife, Atticus knows that he will have problems with Bob Ewell who has evil intentions regarding Tom Robinson. For instance, he is a most uncooperative witness and perjures himself in his testimony.
4. If a person is cut by this rusty knife, he/she may become infected. The attention that Ewell brings to poor Tom Robinson infects the townspeople who talk of putting the blacks in their place such as Mrs. Merriweather; it brings out mobs; it elicits name-calling of Atticus.
5.It is still a knife and can do harm, even when cast aside. After the trial, many felt that people would simply return to their daily lives. However, Tom insults Atticus publicly, but Atticus thinks the act is nothing; then, Ewell attacks his children, injuring Jem's arm badly.