From the day she debuted on Game of Thrones, Olenna Tyrell has been a force to be reckon with. She didn’t give two shits about how much power Cersei had or had not. She did whatever she needed to to protect her family. Her only downfall was thinking that Cersei didn’t have it in her to do the things she did.
Dany’s advisers aren’t the greatest strategists and Lady Tyrell had to pay the price. The Lannister army tore her place apart, but sweet Jaime had bargained out a deal with his wicked sister to let the old broad down easy. Poison, that would let her peacefully drift off. No pain, no mess. Just a drink.
However, Olenna wasn’t one to not get the last word in. After making sure what she was getting would be quick and easy, she dropped the ball on Jaime. She killed Joffrey. Well, she orchestrated the whole debacle and hadn’t realized that the poison would do what it did. She gave the last stab before drifting off to her death, leaving Jaime and eventually Cersei with the knowledge of knowing she went off easy after killing their first born (barf… incest with the Lannisters never gets less queasy).