Sandra Melgar & Other Female Serial Killers

Two days before Christmas, in 2012, Sandra - or Sandy - Melgar was found tied up inside of a closet in her home in a nice neighbourhood of Houston. A chair was forced against the doorknob, keeping her trapped in the confined space. Her husband Jaime - Jim - Melgar had been stuffed into a different closet - but unlike Sandy, Jim had been killed, first. A medical examiner's report concluded that Jim had died of blunt force trauma, and multiple stab wounds that had been inflicted with a kitchen knife. The community was horrified, and their horror only grew when police announced that they'd identified the person responsible for the attack on the Melgar family - Sandy Melgar, herself.
Why did she do it?