re: Crackpot ASOIAF Theories SPOILERS

Crackpot theory: Tywin was also one of the orchestrators of the Purple Wedding. It would have rid him of Tyrion without kinslaying, and Joffrey as someone who would without question ruin the realm, and Sansa had outlived her usefulness after the events of the Red Wedding. He was also saying that Joffrey needed a "sharp lesson".

Perhaps that was part of the deal Littlefinger had to make, in that they wouldn't tolerate Margaery marrying someone like Joffrey, and Tywin had to agree and let it happen, although he was more than happy to let it happen. That would allow Tommen to rule, who could easily be controlled and actually turned into a good king. Plus his monstrous son would either be executed or at the Wall, never to be heard from again.