What quotes in To Kill a Mockingbird reveal Mr. Underwood's racism and bigotry, other than when Atticus says he despises all negroes?

The whole town of Maycomb is aware that Braxton Underwood despises Negroes.

Mr. Underwood is the newspaper publisher.  When Atticus is accosted by a mob of Cunninghams trying to attack his client Tom Robinson, Mr. Underwood holds a rifle on him to cover him in case they get violent.  Later, Atticus makes a comment about this, saying that it is interesting because Mr. Underwood is a racist.

Mr. Underwood demonstrates that he is a racist during the trial by his reaction to Scout and Jem sitting in the balcony.

Mr. Braxton Underwood, who had been sitting quietly in a chair reserved for the Press, soaking up testimony with his sponge of a brain, allowed his bitter eyes to rove over the colored balcony, and they met mine. He gave a snort and looked away. (Ch. 18)

Mr. Underwood does not approve of Scout and Jem, white kids, sitting with the blacks in the balcony.  This is why he snorts.  He likely feels that Tom Robinson is guilty because he is black and accused of attacking a white woman.  He disapproves of the blacks being at the trial in general.

Despite this, Mr. Underwood is annoyed when Tom Robinson is shot.  Black or white, you don’t kill cripples.

Mr. Underwood didn’t talk about miscarriages of justice, he was writing so children could understand. Mr. Underwood simply figured it was a sin to kill cripples, be they standing, sitting, or escaping. He likened Tom’s death to the senseless slaughter of songbirds by hunters and children … (Ch. 25)

The point is that even racists like Mr. Underwood do not approve of shooting a man like Tom Robinson, who is obviously crippled.  The trial resulted in Tom Robinson being convicted although he was innocent, but Underwood doesn’t focus on that.  He just argues that it is wrong to shoot someone who can’t defend himself.