sources I can use to prove pagans did not sacrifice animals?

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a little troubled when I was debating christians on this because they demanded proof for this and it seems hard to find it anywhere.  They were just saying that I am just being squeamish to the history and its like I can get passed killing an animal just fine but like setting up ceremonial ritual and restraining the animal into fear for its life doesn't sound pagan at all, rather psychopathic and subhuman.   I just need some sources, perhaps a book or two or some videos... articles with some actual proof.