I think that Mrs. Salmon experienced a destabilizing effect the moment the Adams brother that had survived was "looking straight over at Mrs. Salmon." Part of the effectiveness of Greene's characterization of Mrs. Salmon was that she was depicted as the unassailable eyewitness. She was able to withstand the glare of the spotlight, did not seek any of the moment for herself, and was able to provide clear and lucid detail as to what she saw on that fateful night. She was beyond reproach as a witness. Mrs. Salmon represented the very essence of what is wanted out of the legal system that depends so much on eyewitness testimony. Yet, when she is unable to distinguish between both brothers, her testimony is in doubt. It is at this moment when she is destabilized. From an instant where she was looked upon as a bedrock and the closest thing to a guarantee, she becomes merely human. It is this level of destabilization that becomes evident when the surviving Adams brother looks at her. The emotion she must be feeling is the pure and stinging feeling of insecurity and doubt. It is an emotion that causes questioning of everything. In the span of five or ten minutes, Mrs. Salmon goes from absolute confidence to pure doubt and insecurity. It is this emotion in both the actual experience of it as well as the transformation from one extreme to the other that would fill her emotional palette at the time of the surviving Adams brother looking her way. It is for this reason that the story ends with a legitimate question as to how Mrs. Salmon would be able to "sleep at night." It is this emotional experience- the set of emotions that prevents an individual to "sleep at night-" that represents what she would be experiencing at that agonizing moment.
The look of the Adams brother would haunt me for the rest of my life and i would be anxious about the killer out loose who is after me for the revenge of getting his brother killed.