At that moment, Harry fully understood for the first time why people said Dumbledore was the only wizard Voldemort had ever feared. The look upon Dumbledore's face as he stared down at the unconscious form of Mad-Eye moody was more terrible than Harry could have ever imagined. There was no benign smile upon Dumbledore's face, no twinkle in the eyes behind the spectacles. There was cold fury in every line of the ancient face; a sense of power radiated from Dumbledore as though he were giving off burning heat.
Never assume that the person you are dealing with is weaker or less important than you are. Some people are slow to take offense, which may make you misjudge the thickness of their skin, and fail to worry about insulting them. But should you offend their honor and their pride, they will overwhelm you with a violence that seems sudden and extreme given their slowness to anger. If you want to turn people down, it is best to do so politely and respectfully, even if you feel their request is impudent or their offer ridiculous.