Mr. Underwood was protecting Atticus from the angry mob that was trying to kill Tom Robinson.
Mr. Underwood is the newspaper man. He values Atticus’s life, and wants to protect him. While he may not want to save Tom Robinson’s life, he would want to protect Atticus.
Mr. Underwood had no use for any organization but The Maycomb Tribune, of which he was the sole owner, editor, and printer. (ch 15)
Mr. Underwood not only runs the newspaper, he lives out of the office. So while the mob was able to get the sherrif Heck Tate out of the picture, and Atticus was unarmed, Mr. Underwood was there to protect him.
Mr. Underwood and a double-barreled shotgun were leaning out his window above The Maycomb Tribune office. (ch 15)
Chances are that Mr. Underwood was not willing to allow violence under his watch one way or another, but he certainly would not let anything happen to Atticus.
Braxton Bragg Underwood, named for a Confederate general, is known to "hate Negroes" and not want them around, but he is not a murderous man. His character illustrates one of Harper Lee's central points: few people are all good or all bad. Mr. Underwood is undeniably prejudiced—but he also does good in the town of Maycomb by supporting Atticus.
Knowing that they have lured the sheriff out of the way, the mob pulls up in dusty cars and one of them demands Tom Robinson. But Atticus refuses to comply and does not move from the door of the jailhouse; the men inform him that Sheriff Tate is not there and there is no one to help him.
Impulsively, then, Scout runs to her father, and Jem tries to catch her. "Go home, Jem. Take Scout and Dill home," he orders his son. But when a burly man grabs Jem, Scout rushes in angrily and kicks the man. Jem continues to refuse to go home. Fortunately, when Scout singles out Mr. Cunningham, he becomes embarrassed and orders the others to clear out with him.
Then, the "soft husky voice" of Tom Robinson is heard. "Mr. Finch? ... They gone?" Atticus tells Tom to sleep, adding that no one will bother him tonight.
"You're d*** tootin' they won't. Had you covered all the time, Atticus." (Ch. 15)
These are the words of Mr. Underwood, who leans out The Maycomb Tribune office window holding a double-barreled shotgun. Since he lives there as well, Mr. Underwood has watched everything, ready to protect Atticus if necessary. He does not want any illegal activity to take place, regardless of how he feels about the black man, Tom Robinson.