Why, according to atticus, does bob ewell bear a grudge?

Answer 1

Because Atticus stripped him of his dignity on the witness stand.

But you'd better actually look at the book and be sure you understand the subtlety of the interaction between Atticus and Bob Ewell, or you'll miss much of the point. A teacher would be able to tell whether you actually read the book and got it, or if you bummed the answer from a friend (like me).

Harper Lee's prose is something to be savored, not squandered. Getting the answer online is only going to cheat you out of an important opportunity to think for yourself and write well about important social issues. You might not believe me now, but my words will come back to haunt you like Boo Radley from behind the shutters. ;-)