"The pellet with the poison's in the flagon with the dragon!
The vessel with the pestle has the brew that is true!"
– Danny Kaye, "The Court Jester"
The King is dead! Long live the poisoner of boy King Joffrey Baratheon, first... and by the Old Gods and New... hopefully the last of his name!
There's been speculation among the Brotherhood without Banners as to just who that poisoner might be.
Actually, there should be no mystery if one watches the peripheral activity during the Purple Wedding.
Re-watch that sequence... if you can tear your attention away from the vicious humiliation Joffrey is inflicting upon his uncle, Tyrion Lannister... and all should be revealed.
Of course! GoT it!