In Season 7, Episode 3 of Game of Thrones, Cersei got revenge on not one but two of the people who killed her children, using their own means.
Sure, the way Olenna died was coincidental, considering Cersei didn't know Olenna was the one who poisoned Joffrey (and she also didn't want to use poison against her). But it worked.
And the way Cersei exacted revenge on Ellaria was even more perfect in its brutality. And not just in the obvious taking-her-daughter-away-with-a-poisoned-kiss symmetry.
You might remember back in Season 6, when Cersei was mourning Myrcella, she was haunted by what would happen to Myrcella's body as it decayed.
By keeping Ellaria alive and locking her in a cell with her daughter's decaying body, Cersei is literally wishing her worst nightmare upon her.
The way Cersei has Ellaria and Tyene chained just outside of each other's reach also strengthens her connection to the Mad King.
Aerys Targaryen had Rickard Stark (Ned's father) tied up and burned alive, while Brandon Stark (Ned's brother) had a noose placed around his neck and a sword placed just out of his reach, so he strangled himself to death trying to get free and save his father. Although we haven't seen it onscreen, it's been referenced quite a few times throughout the show – including several times this season.
Cersei has already shown her similarities to the Mad King in her use of wildfire, and this latest act of revenge further cements it. It could mean Jaime will get around to killing her one of these days – just as he killed Aerys.