MacDonald murdered his wife and one daughter during an argument in the middle of the night Goody. When there are diet pills, depression and lack of sleep involved, I don't think you need a history of bizarre behaviour. Mac certainly had no history of it. He murdered his baby purely because she was there and alive and old enough to say daddy hurt mummy. It's only when you scratch the surface that you discover his history of violant behaviour, and his mental illness. We can't do that with Darlie as no one will talk and she was not subjected to psychological testing that we know of, designed to uncover any hidden pathology. This case is very similar to MacDonald. I believe that Colette was intending to leave Mac and that might have precipitated the argument.
I for one do believe that Darlie and Darin were arguing and not getting along. You don't sleep apart from your husband for days on end just because your baby keeps you awake as he rolls in his crib. How that telegraphs into killing the boys, I don't know.
I for one do not wish to speculate at all as to what Darin and Darlie were all about so that's why I very seldom post on threads like these.