When a person doesn't believe what they're doing is evil, how can you call them evil?
Where is the standard for judgment?
In the past, what Zhang Fei Xuan and the others considered as the standard was determined by the Righteous Path.
It was those who demanded offerings and exterminated Evil Demons that were deemed to be on the Righteous Path.
Of course, incidentally, they also protected mortals, after all, they were under their jurisdiction; if a supreme Great Demon were to emerge, everyone would be doomed.
As for the Demonic Path, take the Sects from Xumi Vein, for example, if they couldn't demand offerings, they would settle for dregs, transforming flesh and blood into the resources needed for their cultivation.
Alchemy, cooking, skinning...everything was available, and what mattered was the reuse of waste.