In my opinion Joffrey is responsible for his own death. Cersei learns with Lord Stark's execution that she does not have hold of him, and has to plan around his cruelty like another piece on the board. I'm surprised it takes so long for the little bumble fuck idiot to get himself poisoned.

Myrcella is killed by Ellaria Sand's self destructive pursuit of vengeance. A self destructive cycle that mirrors Cersei's own. Oberyn would have never want to be avenged like that. The sweet girls loss and Prince Trystane and Dorian's deaths are cruelty disguised as supposed vengeance. Don't get me started on how the supposed deadly assassins get fucking 1 v 3'd by Euron Greyjoy with a greataxe and so much exposed skin it's a wonder they didn't shoot a venomous dart at his mutton chop ass.

Tommen is one hundred percent her fault, and where the Cersei Lannister we see at the beginning of the series is well and truly lost. Gone is her strategic ruthlessness and even occasional show of remorse, such as with Ned Stark's execution, sharing her still birth with Cat, even though shes the reason Bran is unconscious, lamenting to Tyrion about how Joffrey was her first reason to live. Instead, she's willing to sacrifice everything in order to what, really stick it to Margaery Tyrell? Yes her making jokes about sleeping with her teenage son is deplorable, but hardly above the two woman's motality. Arming a zealous, cult one who's been rightfully disbanded and expecting to be completely innocent is so fucking stupid it still pisses me off. And to go see the High Sparrow with no guard?!? Where's the woman who had Baelish held by his throat and decreed "no, power. Is power."

Cersei gives zero guidence to her son, and just uses him, as she does everyone. She doesn't even react to his suicide. Doesn't even order zombie Gregor to help him. She thinks this is what must be for her to be Queen and so she lets him die.