Regarding proving the existence of these spirits and their effects, what evidence do you or they have? Can you lay it out for me so that I can test it myself? If not, then the animals are being sacrificed by people who believe in the existence of these spirits.
That's quite alright for you to think that way. I am personally of the belief that one actually shouldn't try to convince anyone of anything in this regard, particularly on the internet. These are experiences one has to have for themselves. While I would give advice to those fortunate (or unfortunate) enough to stumble onto this path it isn't something I personally advertise about myself outside of this internet talking head and those who have come to know me personally. Nor do I or would I ever claim any particular authority in this matter. Anyone telling you they know the definitive truth about this or any similar subject is a tosser. I have accumulated evidence to my own satisfaction. If I turn out to be cultivating an intense yet functional psychotic delusion at least it was an entertaining one.
What I am trying to represent in the tone of these posts, to the best of my ability, is the perspective of the Palero. These spirits are real. They feel them move, speak with them, and in process of training and initiation systematically test their power and efficiency. If it didn't work they wouldn't have become scratched.. its quite a heavy responsibility and one which continues after you have passed the body.
These people have as much right to think these spirits are real as you do to think they just believe they are. Who are you, the reality police? Which brings us back to the ritual killing issue. Why do the spirits need blood? Because they ask for it, and have traditionally been given it as part of this exchange. They have come to like it and it gives them strength to do work. If that is going to change it is the spirits which will need convincing more than the Paleros.
I understand your objection to thinking of animals as possessions. But the fact of the matter is, in terms of the law, domestic animals are owned. They have been some of our most important and fought over possessions throughout recorded history and well beyond. I think of animal husbandry as a partnership myself. That certainly hasn't always been the case. I see your points personally, but am not sure you could use those arguments to convince a Palo practitioner.