Bro, if he defies to the point of death then what about Noelle? She'd be dead too. THEN he'd be replaced by an even much worse guy(you'll see him soon, he's fucking disgusting) You say you understand he why can't refuse then why are you so confused? Ahuin's been with the saintess for a hella long time, you'd think it would be so easy, just like that? You saw that one panel with the saintess, what do you think was that supposed to symbolize? That was how much influence she has on Ahuin.
Just like HOW MUCH Noelle wanted to beat the shit out of the saintess because Leticia's been hurt again but controlled herself because Leticia ordered her not to. Or how Noelle immediately swore loyalty to Leticia AND the panel on Noelle's thoughts about loving Leticia and compared it to Ahuin's loyalty to Saintess. It's imbeded deeply into their instincts and souls.
And like what Noelle said, if Ahuin will know about Leticia, he'll be lured to their side too. Which
Spoilers
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He will be, he's one of Leticia's first few wings.

i agree w the saintess, ahuin is WEAK-willed, like the dude goes about how he doesnt like the saintess actions and how he loves noelle yet he still follows whatever order hes given, like i understand he technically cant refuse but if hes are really in disagreement he should be able to do something idk even if that something leads to dead, doesnt he have his own morals??
anyways i feel so bad for leticia rn, she really believes ditrian hates her:(