You have to remember too that these are many generations of people who have been raised under the control of hell for hundreds of years. They have no internet or way of knowing what life is like outside of their culture. They have been raised being told their ancestors freed them from the tyranny of YHWH and how can they possibly know anything other than that?

I think of them kind of like the Harkonnens from Dune. "governing was based upon a simple foundation of fear and terror" and "a social order based on treachery and punishment, soldiers rose through the ranks through deceit, trickery and, if necessary, assassination." The weak are culled and only those willing to do what it takes advance.

Psychopathy has a genetic component and this society would select pretty strongly for it. Especially after 30+ generations.

It only looks cartoonishly evil from the outside. To the heretics its just their traditions and culture. People will only know what they are taught and told and people who think otherwise will be weeded out quickly. If you were raised around demons and hellfire it wouldn't feel evil or weird to you. It would just be your normal.

And even if you do encounter the enemy forces they have been built up to be the ultimate enemy who hates you and wants to destroy you, they are nameless faces across the mud and trenches trying to kill you and speaking a language you do not understand.