Lol I understand where you are coming from, but I am guilty of that hypocracy. I think it comes from the circumstances outside of those individual actions. You know Joffrey is a little shit who deserves a slap (at least) for everything he does/has done. Whereas you know Robert has a history of domestic abuse so while Cersei is infuriating in her own right, that slap seems a part of the larger abuse, and so is not cool to me. I do think Tyrion slapping Joff is slightly 'wrong' since he is by our standards a child, but it is hard to see him as one while you are reading.

Just my 2 cents :)