Harry could hardly believe that Dumbledore would not have intervened if such cruelty was happening inside his own house, and yet there was undoubtedly something odd about the story.
As valet, he kept Gerald's bedroom in order, and, as butler, he served the meals with dignity and style, but otherwise he pretty well let matters follow their own course.
The shock and the horror of this idea must've been the most terrible thing in the world and then the question comes: do you follow orders and burn what you've defended, or do you let it be?