What three things does Bob Ewell do that alarm Aunt Alexandra?
After the trial, Bob Ewell threatened Atticus, who made it clear he would not fight even if Ewell attacked. Ewell was also terminated from his job, someone attempted to enter Judge Taylor's house (thought to be Ewell) and the Ewells harrassed Tom Robinson's widow as she walked by their home. Alexandra was especially alarmed by Ewell's threats, his firing, and the harassment of Mrs. Robinson, as they are indication that Ewell has not forgotten about the trial, nor has he forgotten Atticus's role in it, and in fact is still enraged.
Bob Ewell openly threatens Atticus' family after Tom Robinson is acquitted. Bob Ewell spat upon Atticus and threatened revenge.
Bob's comments about Tom Robinson's death were very alarming: "one down and about 2 more to go" is a veiled threat toward Atticus and others who might have taken Tom's side in the case.
He threatens Tom Robinson's widow to the point that her employer has to "pay him a visit" and threaten to involve the law.