Sauros has been executed because of political reasons. If you watched the previous episodes, Rudeus had to hide his Greyrat identity because of some political reasons. Rudeus and his father, Paul belong to the Notos family which is one of the fourth big branches of Greyrat family. Notos and Boreas family has some political issues with each other and the outsiders were constantly trying to dethrone Sauros from his position. It seems they found the Mana calamity as an opportunity to take down Sauros.