So right now I’m thinking about the Night’s Watch of the Game of Thrones/Song of Ice and Fire universe. They’re ostensibly there to fight the evilest evil to ever evil, but
- no one wants the job, so it’s staffed with criminals and outcasts from every corner of the realm who are forced there under pain of death
- the Night’s Watch puts down more petty crime than the evilest evil to ever evil, so few take their job seriously
- it’s in one of the coldest regularly inhabited places, you can’t have wives or children, and you’re stuck there until you die. Most hate the job.
And it works. Most of the Night’s Watch chapter consist of the POV character complaining about it and trying to get the men there to do productive things. No overt preaching — when the POV does get preachy, everyone else is ready to knock him down a peg.
To that end: you can make the job undesirable and/or rarely against true evil and/or rife with issues (desertion, corruption, nepotism, elitism, laziness). Make the characters fight demons within their job as often as they fight the real demons.
Although he’s not a cop per say, Geralt from Andrzej Sapkowski’s Witcher novels might be a good character to look at. The games fall far short of the books, but at the very least The Last Wish is probably worth looking at.
With Geralt, the primary approach is a kind of dispassionate evaluation of the world around him. He’s approaching monsters as “just another person,” and trying to understand the situation before committing. It’s not your stereotypical monster hunting zealot.
The books do get fairly philosophical at times, but that doesn’t come across as preachy, because it’s usually characters debating, rather than one person saying “this how things should be” in monologue.
For a more urban fantasy setting, the Night Watch novels by Sergei Lukyanenko are worth looking at. I’m not a fan, but they are good. The series is more about maintaining a balance between the Night Watch and Day Watch, keeping an eye on all supernatural activity, without really caring about what people are doing, so long as they’re adhering to the setting’s bureaucracy.
I’ve got a soft spot for C.L. Werner’s Matthias Thulmann Witchhunter novels from Warhammer. It’s tie-in pulp, but it’s surprisingly good pulp. It also has some nice contrasts in levels of zealotry presented.
Thulmann’s an interesting flavor of zealot, who is actually motivated by the well-being of the people around him, rather than just using that as a posthumous justification for his actions. In a setting where “cleanse it with fire” is usually the only real solution, where mercy is usually a sign of corruption, and where Witch Hunters do frequently go into the psychopathic range, it has some nice flavor that might be worth looking at.
Of course, if you’re writing characters who are “like cops,” it’s probably a good idea to actually look at cops. Law & Order and Homicide: Life on the Street are fantastic primers for looking at police. Southland and The Shield are also very good to look at. They’re not fantasy, but they do give some wonderful insight into the psychology of people that have to deal with human evil because it’s their job.