Why didn't the sisters tell Mr. Murphy about Gran's treatment of Laddy in the story, In the Zoo?

The girls did not tell Mr. Murphy what was wrong because they knew, from the time a boy squirted his skunk with a water pistol and Mr. Murphy responded by throwing a rock at the boy's back, that he could become dangerously angry. However, Mr. Murphy heard about the dog's transformation, and when he confronted Gran, she released Caesar, who pounced on his monkey and killed it. Mr. Murphy began sobbing and very early the next morning, he poisoned Caesar's meat, killing him.