Why does Aunt Alexandra continue to worry about the threat posed by Bob Ewell?

It is rumored that Bob Ewell hinted at killing Atticus. Word around town said that after Tom Robinson was killed, Bob Ewell said "one down and about two more to go" (To Kill a Mockingbird, Chapter 25).

Helen, Tom's widow, has to walk by the Ewell house on her way to work. As she walks by, members of the Ewell family harass her. Helen starts walking another way just to avoid them. Bob Ewell also starts following her on the road, making crude comments to her. Meanwhile, someone breaks into Judge Taylor's house. Many people suspect that Bob Ewell did it.

When Aunt Alexandra hears about this, she recalls Bob Ewell's supposed threat. She does not like Bob Ewell's treatment of Helen, and she finds it strange that he would break into the judge's house. In addition to these things being wrong, it makes her worry that Bob Ewell is dangerous. She expresses her concerns to her brother about the matter:

"That man seems to have a permanent running grudge against everybody connected with that case. I know how that kind are about paying off grudges, but I don't understand why he should harbor one—he had his way in court, didn't he?" (Chapter 27).

Aunt Alexandra is puzzled as to why Bob Ewell is still angry. After all, Tom was found guilty and sent to prison. Still, she worries that he will harm Atticus. After all, he is bold enough to break into the judge's house while it looks like he is home.