Was, to pick a famous example, Henry VIII’s seemingly-psychopathic willingness to execute wives and long-time friends/councilors seen as such by contemporaries? I’m just wondering why Joffrey’s evident sadism receives such an apathetic reception from the courtiers.

It was certainly seen as capricious. Kings were supposed to execute people, but the rapid shifts back and forth made for a very nervous politics where partisans strove mightily to change the world and paid the price with their heads, and the survivors were those who were best able to turn their coats on a moment’s notice. 

So I think the apathy you perceive is most likely a cover for a good deal of terror, and the majority were trying to keep their heads down and avoid his attentions.