Why would Underwood, who hates Negroes, protect Atticus from a mob that wants to lynch Robinson?To Kill a Mockingbird by Harper Lee

While the answer to what motivates Mr. Underwood in
Chapter 15 of To Kill a Mockingbird to defend Atticus from his
The Maycomb Tribune office window other than to protect an upright
citizen of Maycomb from physical harm is not evident, more explanation to Mr.
Underwood's thinking comes through his editorial which follows "The Colored News" of the
town's newspaper.  In Chapter 25, Mr. Underwood, Scout narrates, is "at his most
bitter."

In his vituperative editorial, Mr. Underwood
expresses his belief, one that coincides with that of Atticus Finch,
that

it is 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, and Maycomb thought he was
trying to write an eitorial poetical enough to be reprinted in The Montgomery
Advertiser.

By pondering
the meaning of Mr. Underwood's words along with what she has witnessed at Tom's trial,
Scout deduces that Mr. Underwood's words are a scathing attack upon the "secret courts
of men's hearts" that condemned Tom:  "Tom was a dead man the minute that Mayella Ewell
opened her mouth and screamed."  After the shooting of poor Tom Robinson, Mr. Underwood
has perceived the darkness of racial prejudice, a bias that supercedes all rationality,
all morality, all fairness, and all justice.