My 2 cents here, if small or large pd's had anything to do with it the criminalists for the Nicole Brown Simpson/Ron Goldman case would have been a slam dunk don'tcha think? To me it seems more plausible that the financial stress and the discord because of it caused her to snap. She would have had to considered it before, the boys' murder. I can see her daydreaming of them getting murdered, gone, the responsibility and care for only one child and all of the attention she would gain from it. She was very emotionally immature and I think from what we have seen from her mother, Darlie Kee, that she came from a very ignorant background.
I also feel like if Darin was the one to have committed this double homicide Darlie would have ratted him out a long time ago. I feel quite sure that Darin knew it was Darlie and did cooperate in helping her to sanitize the scene.