From their standpoint, demons are no more evil than anyone else. It's not the demons' fault that humans are so tasty and respond in such amusing fashions when tortured. It's not the demons' fault that humans are so easily turned into slaves. It's not the demons' fault that human blood, brains, organs, deaths make for such useful ritual components. Nope. The demons are merely harvesting a useful resource. If anything, it's the humans who are evil for opposing the natural principles of might makes right and the strong absolutely ruling the weak.