Scout notes that Bob Ewell "struts" up to the witness stand. When Mr. Gilmer starts questioning him, Mr. Ewell is flippant and Judge Taylor reprimands him for this. When asked to describe what happened, Mr. Ewell inserts a racial slur into an ignorant accusation.
Mr. Ewell’s face grew scarlet. He stood up and pointed his finger at Tom Robinson. “—I seen that black nigger yonder ruttin’ on my Mayella!”
This sends the courtroom into an uproar which suited Mr. Ewell just fine as he sat back "smugly" in the witness chair. Again the judge reprimands him and asks that he use proper "Christian English." The judge recognized Mr. Ewell felt justified in being smug and arrogant, and the judge was not fooled when Bob appeared to be forthright.
Smugness faded from it, replaced by a dogged earnestness that fooled Judge Taylor not at all: as long as Mr. Ewell was on the stand, the judge kept his eyes on him, as if daring him to make a false move.
When Atticus questions him, Mr. Ewell is still smug and thinks he has the upper hand. However, Atticus clearly has the upper hand and demonstrates it by revealing the fact that Mr. Ewell is left-handed. It is more likely that Mayella was beaten by a left-handed person.
Finally, Mr. Gilmer asks Bob if he is ambidextrous to which he replies, "“I most positively am not, I can use one hand good as the other." Mr. Ewell reveals his ignorance again. From his demeanor and language throughout his testimony, Mr. Ewell comes across as a smug, arrogant, ignorant racist who assumes that the jury is already on his side.