After reading that new information provided to MSM by Gma, it appears that SA was behaving in a manner that should have raised red flags. That being said. There are literally hundreds and thousands of cases where women get into bad relationships and they endure such psychological torture by their suitor that they basically believe that they cannot be away from them. It is entirely possible that BD thought that she was protecting HD from SA by staying with him. It would be easy enough to convince her that "if you leave me you can bet your life little princess will regret it!"
I have an aunt that has been in several of these relationships. It seems like no matter what she does she is incapable of breaking the abusive cycle. She seems to be drawn back to exactly the type of men that will continue to do this to her. Those men also are very good at picking out women with insecurities (the are predators, after all) and then exploiting them.
Whoever fights monsters should see to it that in the process he does not become a monster. And if you gaze long enough into an abyss, the abyss will gaze back into you.