I'm guessing "systematically burning people alive who you've brainwashed into believeing they are gods" and "cutting out a guy's eyes and tongue for trying to help the man he's been trained to help all his life", not to mention the way religious dogma crushes/enslaves original thinking and freedom = horror.
I thought the use of "shit" at that point was actually rather deft choice by the writer, as it isn't only a curse word but the most base expression for the material (and "crap" would have felt too modern) - at that point in the story, Aki has renounced his godhood and so his "fecal matter" is now, again, just "shit", like any human produces (and ends up causing his demise. And, I guess to be didactic, I would point out that "breaks the vessel" (of shit) at the same point he's also "breaking The Vessel" (of Godhood).
I don't have any problem with Numa knowing the word - I would imagine everyone else in the temple (including Numa) "shits" but Aki is divine, so he "defecates".