Joffrey Baratheon's death was a major turning point in Game of Thrones, but it's possible that his own grandfather had a part in the deadly event. A theory suggests that Tywin Lannister had intel about the assassination of the King, and allowed it to play out.
King Joffrey's demise came during Game of Thrones season 4 in the episode, "The Lion and the Rose." Following the major wedding uniting House Lannister and House Tyrell, Joffrey planned an ample amount of entertainment for the guests. At one point, Joffrey forced Tyrion into being his cupbearer. He messed around with his uncle before the pair entered a standoff. Joffrey then demanded that Tyrion brought him more wine and he obliged, pouring him a new glass. After drinking the wine, Joffrey started to choke before violently vomiting. Joffrey's skin turned purple as blood poured from his nose. He quickly died from asphyxiation, but lived just long enough to gesture towards Tyrion, pinning the murder on him.
It was later revealed that Lady Olenna Tyrell was the one who poisoned Joffrey as a way to protect her granddaughter, Margaery, from being married to a sadistic leader. One could argue that Olenna saved most of Westeros from Joffrey, but there is also evidence that Tywin was aware of what was going to happen to his grandson. The scene leading up to Joffrey's death seemed to purposefully focus on Tywin when his grandson demanded Tyrion to fill up his cup. Up to that point, Tywin was expressionless, but he seemed to crack a little as if he was anxious for what was about to happen. When Joffrey later drank the poison and began to suffocate, Tywin didn't initially run over to him, but instead went over to Tommen - who would go on to succeed his older brother to the Iron Throne.
So why exactly would Tywin step aside as his grandson lay dying? It's possible that the alliance between the Lannisters and Tyrells were more important to him than having Joffrey alive. He could have had knowledge of the assassination attempt and allowed it to happen to keep Olenna happy. There's also no question that Joffrey wasn't his favorite grandchild. Joffrey refused counsel on a number of instances and had a habit of provoking Tywin. Tommen was young, but he was clearly the better choice as the king in the mind of Tywin. During Joffrey's funeral, Tywin even stated to Tommen that his brother was not wise and proved himself to be a bad king. While Joffrey was choking, Tywin could have seen it as an opportunity to push the current king out to make way for Tommen, someone he had better control over.
Another benefit of Joffrey's death for Tywin was the fact that the murder was pinned on Tyrion. It was no secret that Tywin treated Tyrion differently and that the father-son duo had a horrible relationship. By taking part in Joffrey's murder, or ignoring the plans for the attempt, Tywin could have planned for Tyrion being the sole suspect. After Tyrion realized that his murder trial was just a sham to give his father an excuse to execute him, he instead asked for a trial by combat, in which Oberyn Martell took Tyrion's place and was ultimately killed.
After being secretly released from jail, Tyrion confronted his father and asked why he would pin a murder on him that most people, including Tywin, knew he didn't commit. Tywin claimed that he wasn't going to carry out the execution, but Tyrion had heard enough and killed his father with a crossbow (on the toilet no less). With Tywin dead, Game of Thrones fans will never know the truth - but a lot of signs point to the Lannister patriarch trying to change the future of Westeros by allowing Joffrey's death.