(I am assuming that this question pertains to the novel To Kill a Mockingbird.)
As Jem states in the novel, there is a societal pecking order in Maycomb: Negroes are on the bottom of Maycomb society, while the Ewell famiy ("the disgrace of Maycomb" for generations) is only slightly higher. However, in the eyes of the all-white jury, the trial comes down to a simple matter of black vs. white: Tom is black, Mayella Ewell is white, and their decision becomes a simple one. Since a white man's (or woman's) word is always believed over the word of a black man, the jury's decision is already determined before the evidence is presented.
It is obvious from the testimony that Tom is an innocent man. Atticus determines that Mayella's injuries must have been caused by a left-handed man (Tom's left arm is crippled); and the Ewell's testimony conflicts with each other's as well as with Tom's. Tom is the victim here, not Mayella; Mayella's lies on the stand make her seem like a sympathetic character, but her charges against Tom--which carry a death sentence--instead make her the villain of the trial. But, as Atticus knew before the trial began, Tom's fate was sealed by the 1930s Southern bias that the average white man held against Negroes.
Atticus hopes that the verdict will be overturned upon appeal, but Tom's decision to try and escape, resulting in his death, leaves the conflict unresolved. Bob Ewell, however, is not satisfied with the outcome: Tom's death does not square things with Bob, who has been disgraced and embarrassed on the stand by Atticus. In Bob's mind, vengeance against Atticus and his family is still a necessary ingredient to clear his own family name.