Hesitation before accepting her death

Yes, Martha wasn't the passive recipient of the bullet at all. Walter pointed the gun at her and hesitated, almost as if he were offering her a solution. He was passive about it, as if he wanted to be sure that she wanted it, too. Martha held the barrel decisively and put it up to her chest. She silently gave him permission. The murder-suicide was a mutual decision.

Maybe Martha felt that this choice WAS an act of strength. Their whole lives as individuals and as a couple revolved around guarding the secret. Constant, paranoid fear of discovery and punishment turned him into an alcoholic and her so cold and calculating that she could shrewdly plan others' death to protect her secret. Sam made her see that she was nothing like the innocent young girl he once knew because the secret had eaten her from the inside like a cancer. She and Walter hated each other, but had to stay close in order to protect their secret. In doing so, they were destroying each other and themselves, smothering each other. Even if Martha could get Sam or someone else to free her from her pact with Walter by killing Walter, it wouldn't be a way out because it would just create a new pact of silence with someone else over the murder of Walter. The murder-suicide pact she made silently with Walter at the end seemed the only way to finally be free of the pact of silence that was slowly killing them both from the inside, anyway.