I'm super confused. Lissala is a Lawful Neutral/Evil god who as an Anthema against breaking oaths or subverting authority. Why is a follower of a god that preaches good virtues of a ruler, involved in a plot to overthrow a government. Makes 0 sense. Kind of a giant plot hole and I'm like less than 3 pages into this story.
Here's the foundry breakdown:
Lissala, the Order of Virtue, has changed drastically from the times of ancient Azlant. Her teachings created the foundation of the Seven Virtues of Rule, later adopted by the realm of Thassilon. While those seven virtues held aspects of charity, humility, and love, Lissala encouraged her followers to focus more prominently on teachings of duty and strict adherence to her guidelines. She believed, and taught, that those who followed her teachings would be greatly rewarded.
Title The Sihedron Scion
Areas of Concern Obedience, reward of service, runes
Edicts Work hard and demand others do so as well, cooperate or avoid conflict with ophidian creatures
Anathema Disobey a superior, shirk your duties, destroy a book
Religious Symbol sihedron
Sacred Animal snake
Sacred Colors gold, green
If the imperators aren't doing their job and aren't following the precepts of Lissala, then the guy who thinks he's a superior follower of Lissala may feel he has a religious duty to replace them. He may also be deluding himself and be mistaken. I guess he'll find out when he dies and faces Lissala in her realm.
You see the "unworthy LE rulers above me must be investigated" thing in other Paizo lore bits. PF1 had bit about the true name of a devil that was based in part of the Pit Fiend above him that he investigated and informed on who was breaking Hell's laws and earned a promotion out of it. Various Hellknight orders recognize Abrogail Thrune as the lawful ruler of Cheliax, but they also recognize higher laws of their order and they've dropped plot points in various adventures of Hellknight leaders investigating Abbie or holding suspicions that she's bending her own laws and might need to be reigned in.
Basically not subverting authority only goes so far - you don't have to be a chump/victim/patsy if those above you are obviously disregarding the rules/mission they expect you to follow. Duties are owed to those below you in the hierachy as part of your (evil) rights/privileges.
Xenocrat has covered it well IMO.
But it basically boils down to "if the authority isn't following their own rules or isn't upholding their duties to the people they're ruling over" then they aren't a legitimate authority.