I always wondered this in the book and the movie. Why in the world did she have the guy come at night when the kids were in the house. Why not have him do it when the kids are at school? Makes no sense to me. Her carelessness basically cost the lives of her two daughters and essentially her son. Libby is screwed up forever.
shareI think it was Calvin who dictated when and how it would happen. His all-too-short chapter in the book explains that he'd encountered Runner in the bar earlier that night and taken such a dislike to him that he wanted to frame him for it. He must have heard - either from Runner's loose lips in the bar, or from Patty herself when they met, that there'd been a loud and heated argument at the house earlier that evening, witnessed by her kids and I think her sister too, and therefore doing it that night - just a few hours later - would make everyone point to Runner all the more.
Also it was hammered home to her, both by Calvin and the guy who set them up (Len?) that Ben would need the life insurance money immediately for the mess he was in, and so the sooner it happened the better. As someone's stated, it was still Christmas break from school, they possibly weren't back in until the following Monday, nearly a week later, and it couldn't wait that long to have it happen in the daytime.