From what I've read about Darlie, she seems to be an extremely narcissistic, attention-seeking person. Am I right in saying she once made a false accusation of rape because Darin wasn't paying her enough attention at a party? Then there are her suicidal thoughts...so you have someone who wants attention, attention, attention all the time. Maybe that indicates a desperate need to feel loved, who knows? But she's also suicidal, she's literally self-destructive. A state of mind that can't have been helped by those diet pills she was chucking down her throat...
I think she intended to kill herself after murdering her boys but at the last minute she couldn't quite bring herself to finish the act. The whole thing was meant to be aimed at Darin- "Look what you made me do"...and maybe it's his immense guilt that has caused him to go along with the intruder theory.
Happy 2006, by the way!