How could a mother do such a thing?

My uncle brought to my attention the murder of his ex-neighbors daughter about a week ago. Guess who the murderer was? The mother. She allegedly stabbed her 6 year old daughter 11 times with a knife. My uncle had just recently moved in January and he tells me how sweet the little girl used to be and how he couldn't imagine why the mother would kill her. Just picturing how scared the little girl was laying in agony, breathing her last breaths, knowing she was going to die. What is going on with siblings killing siblings nowadays? I've heard of mothers drowning their children, that guy who shot his family in the car, Chris Benoit strangling his family, now this? I mean, come on. I would never have expected this to happen in such a good neighborhood either. I guess crazy people are everywhere.