Time
She waited up for him. Despite her denial, he might have told her when he would return.
When he came in alone and changed to his smoking-jacket, she knew he was not expecting any late visitors.
She chose a time shortly after his return because it would appear that he had brought someone home with him, and that this other person had killed him.
Means
She used Benson's own gun. Benson undoubtedly had more than one; for he would have been more likely to keep a gun in his bed-room than in his living-room; and since a Smith and Wesson was found in the living-room, there probably was another in the bed-room.
Being his housekeeper, she knew of the gun upstairs. After he had gone down to the living-room to read, she secured it, and took it with her, concealed under her apron.
She threw the gun away or hid it after the shooting. She had all night in which to dispose of it.
She was frightened when asked what fire-arms Benson kept about the house, for she was not sure whether or not we knew of the gun in the bed-room.