But why would Lady Olenna do such a thing? I mean as much as she despises Joffrey, she knows her granddaughters best interest is that he has to live at least until she's pregnant. Now the throne either goes to Tommen or Stannis etc steps back in the game.
Currently she nor her family doesn't gain anything to kill him. They do lose a lot though.
They wouldn't give it to Stannis while Tommen is still in King's Landing. For how much they scheme, I'm sure Olenna would prefer a kid on the throne they could manipulate vs Joffery. I'm pretty sure everyone but Cersei would prefer it for that matter...
Also thank god for HBO's servers not taking a huge shit again this time.