I actually think that the real reason that Mel has been burning people to death has to do more with maintaining her youthful appearance. I was left with the strong impression from her POV chapter that she is much older than she appears, and it seems to me that somehow the burning of these victims somehow helps her stay young looking and hale. I don't think any of these burnings does anything more than that.

Also, human sacrifice (historically) has included the killing of people who were no real value to the sacrificing culture. Look at who the Aztecs, Maya and other MezoAmerican cultures sacrificed: enemy soldiers and villagers. Sure, they could potentially have been good slave labor, but it was more valuable to them as a society to use them as blood sacrifice. Normally, yes the term sacrifice means you are giving up something important to you, but when it comes to human sacrifice, for millenia it has included those who were not important to you (there ARE exceptions to this).