Let's put a face with the story...
http://deseretnews.com/dn/view/0,5143,695242138,00.html
to the right you can see pics of the husband as well.
The article mentions that he shot her in the back and in the head as she was, I believe, walking to church.
In this instance, you can play what ifs, but he was behind her and she likely never knew anything was amiss before it happened. And, it was likely over before anyone could do anything. Which is likely what he wanted, judging from the angle of attack and the thoroughness of his strategy.
Domestic violence is a difficult thing to figure. And, a question of a duty to act may or may not be the best conversation here.
Sometimes your number is up whether you're on the church steps, taking the wafer and wine, fishing with dynamite, juggling grenades, or whatever.
I'm just very sad for everyone involved. Especially the victim, pictured above.
Sorry for talking about my feelings but sometimes it's appropriate to treat a tragedy like one.