From Wikipedia
Motives
Though a motive is unclear, prosecutors claimed that Routier murdered her two sons because she was angered by her family's financial difficulties. Her husband, Darin, a small business owner, and the family enjoyed a relatively high annual income. However, most of the money that he earned was quickly spent - later termed as 'living large' by Darin Routier on June the 14th of 1996 to Joe Munoz of Channel 5. The family lived in a lavish Georgian home in an affluent neighborhood, drove a Jaguar, and owned a $20,000 boat. Prosecutors argue that Routier feared that, with little money left in the bank, her affluent lifestyle was threatened, and killed her children to rid herself of a financial burden. This claim has been disputed by her supporters and family. By the time of the murders, the money had run out...the Jaguar wasn't running, the boat they owned wasn't running, their income was ninety thousand dollars less than the year before, they owed up to ten thousand in back taxes, twelve thousand in credit card debt, were two months behind on their mortgage, and the couple had just been denied a five thousand dollar loan from their bank. At the time of the murder, an insurance policy for five thousand dollars for each of their sons was laying in plain view of the responding officers.
Controversy
There is still some dispute over whether Routier actually committed the crime, and her family has organized a group of followers that feel she should be set free. She is often the focus of anti-capital punishment groups, who use her case to argue that the system is rigged and unfair, especially when trials are conducted based upon circumstantial evidence alone. However they fail to remind the public that, according to her defenders, the knife used in the death of Devon was not the same one used on Damon and Darlie and none of his blood was found on the weapon used. According to the defense, this second knife was never recovered. The experts for the prosecution, including the medical examiner for the county, refute this claim by the defense saying that the knife presented could have easily made the wounds to all three involved and did not eliminate this knife as the murder weapon for either boys.
http://en.wikipedia.org/wiki/Darlie_Routier