In "The Queen's Justice", Jaime Lannister seizes control of Highgarden on Cersei's orders. He confronts Olenna, granting her a painless death by poison, putting it in her cup of wine as she watches.

Why Jaime killed Olenna Tyrell?

However, some are wondering why did Jaime kill Lady Olenna on Game of Thrones? The simple answer is: because Cersei wanted him to. Despite many fans' hopes, Jaime is still very much in love with Cersei, which was proven after they hooked up again for the first time in a long while during Sunday's episode.

How did Olenna get the poison in the cup?

During the feast, the Tyrell matriarch sneakily took the poisoned stone from Sansa's necklace and put it in Joffrey's wine glass while he was mocking his uncle, cutting open his wedding cake with a sword, and just acting the pompous fool in general. The Kingchild drank the toxic alcohol and died - painfully.