Man's ability to find excuses is unbounded
The suffering demon:
- Is just putting up an act to test the determination and holiness of its prospective vessel. "Don't fall to this trap, just to what's necessary. With determined compassion."
- "Needs to be cleansed from impurities" and will be grateful once bound. (Doesn't expect gratitude after being bound, or disobeys? "Cleanse" some more. We all strive for perfection, but even the holiest vessel may overlook some hidden speck on the demon's soul, actually greet any insubordination as an opportunity to make your demon even cleaner.)
Sure we revere the demons in their unbound form, because we revere their holy aspects, but we accept that they, like us, have lower selves in them. "In this never-complete journey to perfection, we're a perfect couple, the spirit and me."
Some people are just cruel and manipulating
It's these that are invited to bind to a demon (others being "not strong enough").
The fact that the demons are being tortured into submission is just never told to laypeople; when a future saint it selected, they are closely monitored for signs of compassion to the demon, and those deemed "not strong enough after all" simply have an unfortunate accident.
This isn't perfect, sometimes people get second thoughts after a while. Well... saints can become heretics and be punished harshly. Or they can be derided - "St. Idiota was a saint alright, but he just didn't fully understand the nature of the binding and actually believed his spirit wasn't willing, and nobody could talk him out of that nonsense - but we was a really kind soul".
Examples of all of this can be found in the reports on Gulag and KZ personnel.
Also, in the way how inevitable moral crises were handled: Those who couldn't stand it were excused and removed from the process, and either would keep quiet or be sent somewhere they could die a heroic death, and quickly, please.
These scenarios can be combined
Even the cruel and manipulative can have compassion, and to silence that, they will invent excuses. From "the demon is weak and fully deserves it" to "I'm just helping the demon become a better being".
Also, these rulers can be nice to the laypersons and cruel to demons, provided the cult establishes a clear ideology why cruelty to humans is not okay; in that case, you'll get a hypocritical saintly elite that may or may not rule, and a significant fraction of them will have a measure of guilt, sometimes conscious, sometimes subconscious. This would make for a very interesting setting if the story is told from a layperson's perspective, because the different saintly people that they encounter will react oh so differently and leave the layperson (and the reader) wondering what's making the saints so different.