grandmother throws toddler off of five story tall walkway

Definitely sounds like a mental illness and just snapped. I'm wondering if the daughter/granddaughter moving into the house with the other grandparents contributed to it, though - like it was bothering her in the back of her mind and had a lot of resentment towards the daughter. And since she was battling depression, I wonder if she was on any meds for it. Sometimes the meds are worse than the disease.