I actually have a lot of sympathy for the mother in this case - dealing with an inconsolable 5 year old and a newborn sounds hellish. And for the child - the disruptions of the new baby and staying in a strange house could well lead to him acting out, which has been compounded by seeing his pet killed in front of him.

But the grandmother doesn't have that excuse and her behaviour does suggest a) a pattern of unfairly blaming the LW and b) over reacting to avoid acknowledging that this could have happened at any time.