It's sad and it's easy to throw blame, but stuff happens. Of course, most of us say that stuff like this will NEVER happen to them, but you never know. I am sure that this man never intended on leaving his granddaughter in the car. Never say never.
I would be more inclined to place ultimate blame on him had he clearly been negligent. Of course, this was negligence, but it's not like he PURPOSELY left the kid in the car while he went and got a haircut, went shopping, or something, like other "parents" have.
I feel sorry for him because he will have a LIFETIME of guilt on his shoulders.
Just pray for all the families.