For Olenna Tyrell to drive that one final mental dagger, even after Jaime informed her of the mercy he worked out for her, was a great Game Of Thrones moment, and a righteous exit for Diana Rigg’s crafty avenger.
Indeed, Lady Olenna Tyrell did have the last laugh when she revealed to Jaime Lannister that she was the one who poisoned Joffrey, and that she was telling him expressly because she wanted Cersei to know.
Cersei likely will be fuming when she finds out that Jaime granted Olenna the mercy of a painless death, although considering the torture Cersei had planned for the elderly matriarch, Jaime might opt not to tell the Queen anything of the sort.