During the trial, Bob Ewell is kept firmly in place by the presence of Judge Taylor, who instructs him to "keep [his] testimony within the confines of Christian English usage, if that is possible." He doesn't get much opportunity to threaten the Finch family there.
Although the Ewells are successful in sending Tom Robinson to jail without any cause, Bob Ewell is not happy with the outcome because Atticus, through his unwavering perseverance for justice, has shown that the Ewells are lying.
Not long after the trial, Miss Stephanie is a bit too excited to share with the Finch children that Bob Ewell has stopped their father on the street, spit in his face, and threatened revenge "if it took the rest of his life." Although the reader receives this information indirectly, it seems to be accurate because the children go straight to Atticus to ask him about it. Atticus tells them that this is true and that he believes Bob Ewell meant what he'd said. However, Atticus says that if he has saved Mayella from a beating by taking some of Bob's venom, he is glad to take it. The contrast between Atticus and Bob is striking here.
The full portrait of Bob's evil lies in the fact that he follows through on his threats. While he doesn't come after Atticus directly, he does try to murder his children. Atticus believed Bob's evil threats but likely thought Bob's evil revenge would be directed at him—not at his innocent children.