I’m sure you’re right that many torturers did enjoy the job, just like today, but history shows that many people did pass the test, and were not boiled or burned. The easiest explanation for non-believers is that the test was rigged.
Although it varies wildly with time and place (just like today) Medieval punishments weren’t uniformly, grotesquely more harmful than modern ones. That’s just something that modern authorities want you to believe in order to excuse their own routine cruelties like waterboarding, life imprisonment, institutional rape, &etc.
Although this no doubt happened from time to time, I believe it’s more common to become an atheist in such circumstances. Satanists haven’t lost their faith, they’ve just transferred allegiance to a different character in the same mythos.