Why does Mr. Martin buy Camels, even though he has never smoked?

In "The Catbird Seat" by James Thurber, Edward Martin buys a pack of Camels as part of a plan to kill Ulgine Barrows.

When Martin purchases the cigarettes, Thurber says that his coworkers would not have believed it if they'd seen it, because he was a known non-smoker. No one notices the purchase he makes, however. He leaves "the most crowded cigar store on Broadway" with the cigarettes. Martin is planning to leave the cigarette...

In "The Catbird Seat" by James Thurber, Edward Martin buys a pack of Camels as part of a plan to kill Ulgine Barrows.

When Martin purchases the cigarettes, Thurber says that his coworkers would not have believed it if they'd seen it, because he was a known non-smoker. No one notices the purchase he makes, however. He leaves "the most crowded cigar store on Broadway" with the cigarettes. Martin is planning to leave the cigarette near Ulgine's dead body so that the police will believe a smoker committed the crime. 

Barrows is the special advisor to the President of F&S, where Martin is the head of the filing department. Her time there has displeased Martin, and he sees her as a danger to the order of the company.

When he is about to enact his plan, he changes his mind. Instead, he falsely tells her that he's planning to kill their boss. He does so in a wild, flamboyant manner that is entirely unlike himself.

When Barrows reports it to her boss, the man doesn't believe her. Martin is known for not smoking or drinking and for being a very mellow and staid man. He dismisses her from the company, and Martin gets what he wants without having to commit murder.