Hesitation before accepting her death
I found it interesting and indicative of the theatrical taste of the times that both of the Barbara Stanwyck characters from this film as well as "Double Indemnity" behave in the same manner just before letting themselves be shot to death by their partner in crime. For some reason Martha just ultimately gave up and abandoned her strong willed and manipulative ways when she glances down at the gun Kirk Douglas has aimed at her and does nothing to prevent her demise. Her natural insouciance towards any guilt that had never been of any concern to her before suddenly vanishes and she allows herself the ultimate punishment.
It's funny but her character does the same thing in "Double Indemnity" even though the book never told that story and it ended completely different than the movie. After shooting Fred MacMurray once without successfully finishing him off she let's herself be killed by him for a host of reasons, not the least of one being her last minute falling in love with him for his matching bad" ways to hers.
Martha Ivers also had that last minute realization that she had thrown in her lot with the hopeless alcoholic weakling for better or worse and thereby allows him to perform the coup de grâce without resisting since she was also inextricably tied to him as her partner in unforgivable crimes for which she needed to be punished.