Why does Atticus say Bob Ewell bears a grudge in To Kill a Mockingbird? Who does Ewell see as his enemies?

Atticus says Bob Ewell’s pride was hurt at the trial and he had to take it out on someone. 

Bob Ewell is embarrassed when he finds his daughter with Tom Robinson. To save face, he accuses Tom Robinson of rape, knowing Robinson will be convicted because no one will ever accept a black man’s word against a white man’s. Bob Ewell does not care about how this affects Tom Robinson and his family. Tom Robinson was near his daughter. The racist Bob Ewell would be horrified to see that. 

Atticus humiliates Bob Ewell at the trial by exposing his family situation and proving Bob beat Mayella. Atticus also clearly shows the Ewells are lying about the rape. Tom's conviction does not placate Bob Ewell. That is why he spits in Atticus’s face. Nonetheless, Atticus tells his son,

Jem, see if you can stand in Bob Ewell’s shoes a minute. I destroyed his last shred of credibility at that trial, if he had any to begin with. The man had to have some kind of comeback, his kind always does (Chapter 23).

Bob Ewell considers anyone involved on the defense side of the trial an enemy. This includes Atticus, of course, because he made a fool of him. It also includes anyone Bob would have thought looked down on him. That would be just about the entire town of Maycomb, but specifically anyone involved in the trial. 

Tom Robinson was an enemy, of course. That is why Ewell celebrated his death. 

Maycomb had lost no time in getting Mr. Ewell’s views on Tom’s demise and passing them along through that English Channel of gossip, Miss Stephanie Crawford. Miss Stephanie told Aunt Alexandra in Jem’s presence. . . that Mr. Ewell said it made one down and about two more to go (Chapter 25). 

When Bob Ewell was fired from the WPA for laziness, he “openly accused Atticus of getting his job” (Chapter 27). Ewell’s other enemies are Judge Taylor and Helen Robinson. One night, Judge Taylor thought he saw a prowler. Link Deas, who gave Helen Robinson a job, notices she walks the long way to work. He asks her about this and finds out Mr. Ewell harasses her as she walks by his property. Link Deas tells Ewell off for this. That probably added Link Deas to the enemy list. 

Atticus explains why Bob Ewell was being such a nuisance:

It might be because he knows in his heart that very few people in Maycomb really believed his and Mayella’s yarns. He thought he’d be a hero, but all he got for his pain was. . . was, okay, we’ll convict this Negro but get back to your dump. He’s had his fling with about everybody now, so he ought to be satisfied (Chapter 27).

Atticus was wrong. Bob Ewell saved his worst enemy for last. When he tries to attack Atticus’s children, however, Boo Radley sees him. Boo kills Bob Ewell and saves Jem and Scout.