How to dispel worries?
Only by killing.
With so many blasphemous and evil Living Spirits before her, it would be a miracle if she could resist the urge to kill them.
An Su had a good grasp on Cersei's psychology. To deal with Dianpo, one must adopt the method of a madman.
If Cersei didn't act, it would reveal her so-called justice as fake and hypocritical. If she did, An Su would have successfully manipulated her.
Moreover, Dianpo certainly wouldn't be able to hold back.
An Su deliberately provoked the Azure Sorceress to create this perfect stage for slaughter.
The Azure Magic Eye's full-powered and unrestricted ballot-stuffing seemed invincible, but one just had to extinguish them before the Living Spirits could vote. Eradicating so many in a short time was theoretically impossible, even for An Su.
But every craft has its masters, and in the field of eradication, experts exist.