Why does Mr. Martin change his murder plans?
Mr. Erwin Martin has decided that Ulgine Barrows must be murdered, and although he has spent a great deal of time thinking about it, he has no definite plan on how to accomplish the act. He buys the pack of Camel cigarettes simply as "a small red herring" to cover his trail; he might need "a few puffs" to relieve himself after the murder has been completed. But he carries no murder weapon; his "plan" calls for him to find something to use once he reaches her apartment. He knows there will be a "great risk" of being caught. He spots a poker, andirons and what may be an Indian club. None are satisfactory. A nearby metal knife is insufficient. But when Barrows brings him the scotch and soda,
... it was all to grossly improbable. It was more than that: it was impossible. Somewhere in the back of his mind, a vague idea stirred, sprouted.
Although a bloody murder had been planned, Martin suddenly realizes that this entire scenario--sitting in her apartment with a drink and cigarettes--was all too unbelievable, even to him. Quickly realizing that it would also be too unlikely for anyone else to believe, Martin improvises a foolproof plan that will not include the possibility of breaking the law. By adding a few more implausible events--the threat making a bomb and murdering his boss, and being "coked to the gills"--Martin suddenly envisions the end results. Barrows may not be dead, but she will be out of his work life for good.