You just need to adopt the right viewpoint and then convey that in your writing.
If it is worshipped by a common religion, you should just call it a god. Your people are not really worshipping demons they just have a polytheistic religion with lots of gods of varying power levels. That people indoctrinated to a monotheistic religion would call those gods demons or angels is not really relevant and has no place in your writing.
Wide spread human sacrifice for divine favor is not really any more morally evil than declaring a war to further your national interests. That we think it is, is simply a consequence of our common religions rejecting it while making allowances of convenience for warfare. Since we have for millennia looked to religion for moral guidance, we have a tendency to assume religious dogma defines what is good and evil even then the actual reasons the religion rejects or supports something are not directly relevant.
So you should just deal with sacrifice like we deal with wars. Or like people deal with sizeable portion of the population being in prison or state convicting people to death penalty. Depends on how the sacrifices actually work. We accept people being killed for "greater good" just fine. No reason your people should be any different.
You just need to convey to readers why the people think the sacrifices are the "greater good" and acceptable. And yes, you can use real world as inspiration. It is kind of unsettling but there really is not that much difference between the internal logic of systematic human sacrifice and things we accept just fine.
An example is that people dying are either seen as outsiders (enemies at war, terrorists, criminals, or even just far away foreigners) or heroes sacrificing their lives for all of us. Works for wars or fighting crime (not a coincidence that similar vocabulary is often used) and it would work just fine for human sacrifice as well.
And, while mentioning this is usually a bad idea, it is kind of relevant and should be fine at the end : Nazis actually did just fine at marginalizing millions of Jews, communists and homosexuals and then eventually removing them from society entirely.
Good and proper people have natural tendency to look to religious or ideological authorities to define what is right and wrong in society. So there is really no real reason why people in a society that practices human sacrifice would even think about it beyond it being the "right thing" to do. You just need to marginalize or remove all moral authorities that might disagree.