He had never in his life seen his father so angry. 

"Despite my strict orders not to feed, you were determined to do as you pleased, were you not?  Now look at what you've caused."

The younger man stood proudly, but on this occasion he could not meet his father's eyes. 

"The carelessness you've shown in directly disobeying me, along with your eagerness to feed, has created a blood sickness in the villagers. It will likely spread quickly and kill them all before long. Before we've gotten what we need from them."

He grabbed his son by the collar of his shirt roughly, causing him to stumble forward, nearly falling into the other.

"Do you now see why I insisted on your patience in these matters? If the humans become ill and die, we will be without a sacrifice. An offering in poor condition would be seen as the gravest insult to our creator. You and the others you have foolishly incited into action are far too young to remember how fearsome he can be when displeased. We can all be easily replaced. All of us. If you were not my son, I would have killed you already."

He struck his son hard in the face, much harder than he ever had before.  He turned and walked away before he could see the younger man rub his cheek and smile wickedly.

"But you are wrong, father," he thought. 

"Very soon, it will be I who kills you."