Board Thread:Questions and Answers/@comment-73.157.83.2-20160628012312/@comment-4115045-20160630203651

The answer was given in the television series quite clearly.

The mad king had placed several large quantities of flammable liquid under strategic objects in King's Landing for the occassion that King's Landing was threatened to fall.

He would then order to burn the entire city to the ground with anyone in it. Cirsei had her guards investigate the rumor that those stockpiles of flammables where still there and use the one under the Sept to wipe out the enemies she had, including the Tyrell successors. That led her son to suicide as he then realised what a monster of a mother he actually had that would not stop to hurt, maim or kill anyone in her quest for ultimate power and revenge.

All of this information was given in small chunks during 7 episodes in the sixth season of GoT.

However, what the television series does not show is that such major fires in dense city structures like King's Landing in the days which this series plays in could not be contained. There are several examples of such big fires leading to the destruction of tens of thousands of buildings and hundreds of thousands of people dieing. So while she might have succeeded in taking down her enemies, Cersei actually would have completely destroyed King's Landing and the entire area surrounding it, depending on the wind direction.