All his reasoning is based on maybes, speculations and uncertainties. He is a hypocrite who encyclopedically commits every atrocity in the book while the gods exist
which he thinks other people will commit if they find out they "don't". There's some heavy-duty cognitive dissonance happening here. Ask Durance if Magran isn't "real" or Eder if Eothas isn't. Engwithans say that there's no reason to exist without the gods, but this is absolutely ludicrous and contrary to real life existence in general. Why do they think this? How did they come to that conclusion as a whole civilization? Why would they react this way? They invented a ridiculous solution to an imaginary problem and left only Thaos to clean up, but literally nobody would believe him even if he flat out tells people they are man-made. Even if they believe him, it wouldn't matter.
Have you changed your own perspective in any meaningful way on Eora's gods now that you know they are man-made? How would you react if someone told you that the Christian God really is a bearded man in the sky and he exists 100%, but he was made in Egypt 4000 years ago? The whole concept is mind-blowingly missing the point that I don't have the words to describe how unfathomable such a thing is. There is nothing here that suggests Thaos isn't a raving lunatic who is obsessed with his perceived duty and only does it because he likes the power and influence, but we don't even know this because we don't know who he is as a person, so everything he does seems nonsensical.