Why does Cersei have to go full Bond villain on Tyene and Ellaria?
Look, Cersei cares deeply for her children, who are all... dead. So. Yeah. Now she just wants to watch the world burn, preferably while she's lighting the matches and dropping them into strategically placed reserves of wildfire. Ellaria killed Cersei's daughter Myrcella -- in Dorne because she was supposed to marry Trystane Martell in the old family swap between the Martells and Lannisters that really hasn't worked out well for anyone -- by dosing her with slow-acting poison delivered via goodbye kiss.
Cersei, never one to miss symbolic opportunities, has her creepy right-hand, ex-maester Qyburn cook up some of the stuff. Once Cersei kisses Tyene, the poison starts working. The effects could happen instantaneously, take hours, days, or even weeks. Ellaria will have to watch every second of the grueling slow death. But is there a hope for escape beforehand? Like Goldfinger and his 007-slicing laser beam, Cersei can't resist a weapon of intimate destruction that leaves just a little wiggle room for escape. Remember: Yara is still out there, waiting to slip through the Lannister faction's grasp and save the day. Or maybe Dany swoops in with those dragons after all. Hell, Jon Snow could wind up leading the charge into King's Landing. Point is: don't count the Sand Snakes out completely.