why does a man like mr. underwood cover for Atticus in jail?

Mr Underwood is well known to be a "nigger hater". Beyond his many character flaws, he still respects Atticus and knows that Tom is an innocent man. He was not going to let anything happen to Atticus on his watch. Mr. Underwood writes his articles in his newspaper as he sees things. After the trial we can see a change in Mr. Underwood. Mr. Underwood writes a beautiful metaphorical piece about Tom likening his death to the innocence of songbirds (hence the whole Mockingbird theme).