Cersei's corner was empty. Even King Tommen wouldn't take his mother's side against the High Sparrow to allow the Mountain to fight for her in a trial by combat, so she needed a new plan to avoid being punished for her crimes. Margaery Tyrell was responsible for Tommen's affiliation with the religious zealot and his fanatics, so Cersei made sure she and her brother Loras were among the victims when she ignited wildfire to watch all of her enemies burn.
What Cersei didn't expect was for her last living son to be so anguished by the sight of this devastation he'd end his own life, but as they say, a Lannister always pays her debts. The price of Cersei's power was exactly three children, just as a fortune teller once prophesized.
She's now Queen of the Seven Kingdoms, and even her brother-slash-lover Jaime looks concerned at her coronation. He became a Kingslayer the last time someone was crazed enough to use wildfire like this, and their children were the most humanizing part of her, so now there's no telling what Cersei's capable of.