Wonder why grandparents would kill their grandkid?

https://www.yahoo.com/news/grandparents-arrested-murder-french-toddler-133035787.html
I remember there was a case in Virginia where a grandmoo tossed her grandkid over the rail in a parking garage a few stories up because she was against her daugher's choice in men.

According to the article the grandfather has a history of violence and sexually assaulting children. I wonder which of the grandparents did this to the child? I wonder, indeed. It's a real head scratcher!

I bet he did just that and tried to get "rid of the evidence" and grandmoo is completly depended on him