Why did Cersei blow up the Sept?

Question

I get that Cersei hated those religious extremists who made her parade naked in the city, but what was the point of blowing up the entire Sept? It killed lots of innocents (surrounding buildings also got destroyed). The Great Sept of Baelor building also had historic significance. And if she didn't care any of this, at least it was a total waste of precious Wildfire which could be used in wars.

She could make the faith illegal. She had access to powerful armies and royal guards which could kill all the extremists one by one. Why didn't she choose this option? Why did she go for Mad King's signature move which could spark more rebellion?

Answer 1

I'll try to take your points one by one here...

• what was the point of blowing out the entire Sept?

  She had all her enemies in one place. The High Sparrow, the Sparrows, the Tyrells, etc. What was lost in the show is that the crown owed the Faith A LOT of gold, which is how the Sparrows came to power to begin with; crown a new High Septon who will forgive your debt.

• It killed lots of innocents (surrounding buildings also got destroyed). The Great Sept of Baelor building also had historic significance

  Since when as Cersei cared about innocents? Remember this was woman who was going to have all the highborn maidens killed by Ser Illyn if the Battle of Blackwater was lost not mention going to poison her own son. Would have seen her own brother (Tyrion) killed for a crime he did not commit. Plotted the demise of her own husband. Plotted the initial demise of House Stark. In the game of thrones, you win or you die...

  As far as the building goes, this is I think a call back to the books in which she burns down the Tower of the Hand after Tywin is killed.

• And if she didn't care any of this, at least it was a total waste of precious Wildfire which could be used in wars.

  As far as she knew the war(s) had basically been won at this point. The North has been demolished and taken control by the Boltons (whom at this point still was assumed to be loyal to the crown). The East was held by Littlefinger (whom at this point still was assumed to be loyal to the crown). The South was either the Tyrells who have a stake in the crown and Martells who were on her hit list but not a military power. The Iron Islands have actively sided with her now that Euron has taken over. Daenrys is still in Essos and assumed a non-threat. Jon Snow is still a bastard of the Night's Watch and not a threat.

• She could make the faith illegal. She had access to powerful armies and royal guards which could kill all the extremists one by one.

  Not exactly... Remember this is the same faith that had the power to imprison her in the first place. Cersei is not Queen, just Queen Mother. Tommen and Margarey held the true power. The Kingsguard even rode to the gates of the to forcefully take back Cersei if needed but were ultimately told to stand down by Tommen. Once she is out of prison, Tommen basically has her relegated to her chambers. She is not in any real position to give orders except to maybe the Mountain. Which as a much of a monster as he is probably couldn't take on hundreds of armed sparrows.

• Why didn't she choose this option? Why did she go for Mad King's signature move which could spark more rebellion?

  Well, who could say she actually did it? It was merely a tragic accident from long forgotten things done by the Mad King (hey remember when us Lannisters helped overthrow that guy?).

  Out-of-universe, it shows the viewers that yes Cersei has gone fully mad, as mad as the Mad King was.