Who Killed King Joffrey?
The quick answer for who is responsible for the murder plot of King Joffrey Baratheon would be “The Queen of Thorns” Olenna Redwyne and Lord Peter “Littlefinger” Baelish.
Littlefinger uses Ser Dontos gratitude from being saved by Sansa Stark. He gives the Ser a crystallized version of a poison called “The Strangler” smuggled in by Olenna Redwyne. In the novel, the poison is in a hair net. For the show, Ser Dontos gives Sansa Stark a necklace before the wedding. The necklace had 7 crystals
At the wedding when Olenna Redwyne comforts Sansa Stark, she plays with her necklace and one of the stones is gone. She also makes a crack about killing at weddings.
The poison is placed in King Joffrey’s wine goblet well in reach of Olenna Redwyne.And that’s how Joffrey got himself murdered.