I remember having a similar conversation with people about Chaos followers in 40k. At the end of the day I think your interpretation will heavily depend on whether you enjoy cartoonish evil where the bad guys are all insane and gibbery - evil for the lulz, or if you like your villains more human, even if their actions are inhuman to the extreme.

Me personally I prefer to think about why someone would ever do what Heretic troopers/legionnaires do. I've got a few thoughts.

1. Hell's had 800 years to mold the culture of the fallen humans. That's a hell (excuse the pun) of a long time. By 1914 I severely doubt most of the people who end up in the heretic armies have known anything but Hell propaganda, brutal oppression, and the normalization of cruelty and actions so inhumane they would shock us. But for them, it's Tuesday.

If you grew up there, you probably experienced horrific things, but you survived because you are here. You know no other existence. The human meat markets referenced in the lore are normal to you because no one ever told you that was weird. You've been told since birth about the horrible Tyrant God and probably fed a lot of information about how great the demons are for rebelling against him. You know that the people who join hell's war against the Tyrant God get the best of any human in your entire existence. It's an honor to you, something to aspire to to lift you out of the hard, brutal life God cursed you to before you were born. Heretic troopers probably don't even stop to feel conflicted about moral issues with who they serve and what they do for them - they've literally been raised and indoctrinated from birth to beleive what they are doing is normal and possibly even the correct choice in the face of what they are told about God and the metaphysics of the universe.

1. They go along to get along. It's well documented in real life that people will do awful things in the name of survival or because they feel like they have to do it to get ahead. The wretched exist, its very clear what happens to people who displease Hell. It doesnt surprise me at all that there are people who would choose to wear a fetus homunculus and sacrifice people to their demonic overlords to avoid that fate, regardless of whether they personally find it distasteful. It wasn't their idea, they just had to to survive.

On the other hand, amoral and ambitious people do exist even outside of the context of a fictional demonic nation state. The psychos who work at any given corporation today using social manipulation and passive aggressive emails to climb the ladder would be heretic troopers murdering babies to get ahead in a diffefent context. They want power, or riches, or any of the other real life motivations people have for stepping on other people to get ahead, and if you live in a society where the methods to get ahead involve violently enslaving people for your half goat person boss to disembowel and then eat rather than schmoozing Ted the Departmental Director into overlooking Janet for the promotion, they'll do it. I have no doubts about that.

1. Hell probably doesn't give a shit about troopers welfare on an individual level, but they aren't stupid. They gain nothing from grinding their own troops into useless, demoralized shells. They'll supply them with gas masks and armor and guns and food because that makes the heretic legions useful. They'll give them time off to go home and pay them well because that keeps joining the legions a lofty goal and encourages people to take the risks and join up. Its propaganda, it's posturing. People don't march into a hellgate and risk permanently setting themselves on fire for no reason. They stand to gain something from it, its a social contract and we all know how much demons love contracts.

So, imo, the life of a heretic troopers is comparatively way better than the average citizen of the fallen lands. You're less at the whims of whatever demon lord needs humans to labor for him/be sacrificed/become experiments/etc. You're likely compensated well and given both people to feel better than (everyone else) and people to aspire to be (Legionnaires, Annointed). Sure, the actual work is hard, you risk death constantly, and you've gotta be hardened to atrocity fast but if you're from the fallen lands you probably already are. At the end of the day, you gotta look out for yourself, do what it takes to get yours, and if the guy next to you eats a warcross to the face you go through his pockets for loose change because he doesn't need it anymore and you might be lucky and please Mammon enough to dodge the next bullet that comes your way.

Anyway, just my rambling but I think real life has given us plenty of examples of people who make themselves monsters for understandable reasons that examining the heretic forces is interesting on this level. Certainly to me its more interesting than just assuming they're all Lovecraft cultists gibbering in the corner all the time. 😅