Death and ritual sacrifice are very large components of my magical and religious practice, and I find that very seldom are they explored with any deep understanding of why they are useful and necessary for certain rites and practices.
There is definitely a repugnance for the sacrifice for animals in the frankly White-er side of Western magical and religious practice. I've heard a number of persons rail against it as cruel and entirely unnecessary, that it is uncivilized and barbaric. Obviously I think these persons aren't examining their views deeply; rarely are these individuals persons who abstain from eating animals...they just don't get their hands "dirty". I never hear them complaining about how Jewish folk practice kosher religious slaughter, or even much about the Halal practices of some Muslim folks. Read a newspaper in any city with neighborhoods that have a decent amount of ATR practicing folks and you'd think the death of a chicken or goat was some rare horror, that only occurs during "Satanic" rituals and the like.
Of course, it can certain be done in an inhumane fashion...like most things. It's cruelty that I feel is rightfully opposed, and not animal sacrifice itself.
Properly done, animal sacrifice combines sustaining the spirit and sustaining the community, in addition to taking responsibility for taking and giving life. It places the priest/ess at the center of the Yowa, in the image of Nzambi. The blood of the sacrificed animal and the energies of it's life feed the Nkisi, and it's body is prepared and fed to the Kanda, the community. So, everyone eats of the same flesh, that has been blessed to improve the health and well-being of the entire community. I'm sure this sounds familiar to the Catholic rooted folks.
This animal, instead of dying purely to sustain the body, instead dies to sustain the body and Spirit. I can't think of a better way to honor and respect the spirit of the animals that we take. We use every bit of it, ideally--the menga feeds the nkisi, the edible flesh the people. The inedible bits become ngandos, ritual components with particular powers that are used in various rituals including the building of new nkisi.
Compare this to the animal raised in a factory farm, whose body is wasted and discarded, pumped full of poisons and maimed for pure profit. How is this better for the animal than a clean and spirit-minded death?
I've also heard it argued that for magical purposes the deaths are unnecessary. This is sometimes true; when it comes to feeding spirits there are many offerings one can make instead of the life of an animal. Sometimes the nkisi even prefer these offerings....but you cannot build nkisi without ngandos. You need the nature of the animals eventually, and there are times when simply nothing else will do. During initiations, cleanings, the menga must flow. There is a frisson created with the death and dying of a thing, a door opened between the unmanifest and the manifest. This "energy" makes it easier for the dead to speak and manifest, there is a release of vitality and power that cannot be had in any other way. Responsible killing is part and parcel to a Tata Nkisi's work.
It also has a transformative effect on the person doing the killing; the first animal I took as a sacrifice was intensely personal. I could feel it's heart beating in my hand, and it looked back at me and was intensely present and aware in that moment. I was surprised to find it was difficult; I've eaten COUNTLESS animals over the course of this life, and rarely thought twice about them as entities. This living thing's life was beating in my hands--soon beating it's last--and I was taking it for my own ends, no bones about it. Makes you take a close look at what you're trying to accomplish, especially when it comes to sorcery. Can't take a life for some frivolous bullshit. This bird looked me dead in the eye, and I looked right back at it in surety that the work I was doing was worthwhile. If it hadn't been, I'd have had to let the thing go and live it's life. When you respect the spirits of the animals you work with, religion / sorcery is truly a matter of life and death, and you cannot spend their lives needlessly.
This is similar to the concept of licensia, having the license to do a thing in the religion. You need both the spiritual license to do a thing as sacred and holy as taking life, and the balls to take responsibility for the harm you cause and wield the knife. Don't sit down at your dinner table eating that bacon, and then look askance at me when you've never wielded the knife.
This lesson ends up applying in many areas of life.
I hope the day finds you all well!