The author may be using the "flip the bird" expression to present a graphic image for the reader of what might have been going through the mind of the now dead victim while his wife hesitated to either help him or call for help.

The physical expression is a silent way of expressing anger. So your writer is taking the case where the speaker has not chosen to not speak but was rendered silent by circumstances. Did he chose the last mode of expression available to him before he was killed? Or should he have? That's the ambigous tension being presented in my opinion.