On Game of Thrones, Joffrey was poisoned and died at his own wedding. Tyrion was arrested for the murder, but was he actually the perpetrator? Tonight during Episode 4, "Oathkeeper," it was finally revealed who’s really guilty of regicide.
As most viewers have already come to believe, Tyrion didn’t kill Joffrey — it was a setup. The real killers were Olenna Tyrell, and Littlefinger. You didn’t forget about Littlefinger, did you?
Joffrey’s murder was an elaborate plan that began in the Season 4 premiere, when Ser Dontos reappears to give Sansa a necklace. Unbeknownst to Sansa, one of the jewels on the necklace is actually a vial of poison. The only person who knew is Olenna Tyrell.
During the wedding, Olenna approaches Sansa to apologize for the death of her family at the Red Wedding. During the scene, Olenna fondles Sansa’s necklace and discreetly palms one of the jewels, and in the next scene the jewel is missing from Sansa’s necklac. It’s subtle, but it’s definitely there (or, rather, not there). Olenna even foreshadows what she’s about to do, saying, “Killing a man at a wedding... horrid. What kind of monster would do something like that?” She’s a sly old bird.
Then, while Joffrey is choking, Ser Dontos approaches Sansa and tells her to come with him if she wants to live, knowing that her necklace could implicate her. Dontos takes Sansa to a boat, where Littlefinger is waiting for her. He explains to her that he hired Dontos to give her the necklace with the poison, which was then taken by Olenna to poison Joffrey. Littlefinger then kills Dontos in order to tie up loose ends before heading off to the Vale with Sansa.
What was the motive? Well, Olenna wanted to be rid of Joffrey because he’s awful, and would have abused her granddaughter. It probably also helps that Tommen is young and more impressionable. But why did Littlefinger set it up? Well, because he’s Littlefinger, and who knows why Littlefinger does anything? He does tell Sansa that Joffrey was too unpredictable a friend to have around, so he must feel the Tyrells are a bit more reliable.