From the Llewellyn Magick Blog, August 31, 2015:
The subject of the use of blood in magickal rituals arises every so often in modern occult discussions. And, as you might expect, it tends to be a rather polarizing force. Some are willing to use blood—either their own or that of an animal—in their sacred rituals, while others consider it animal cruelty and are vehemently against the practice. Sadly, these discussions usually get off on the wrong foot from the very beginning, thanks to a very distorted view of ritual sacrifice held by Western culture. Most often, I see the assertion that ritual sacrifice is a method of “powering your magick via the death of a living creature.” And, even more unfortunate, it is sometimes said the magick is powered specifically by the terror and pain suffered by the creature.
I have addressed the subject of ritual sacrifice in many places—see Secrets of the Magickal Grimoires chapter four (Llewellyn), my introduction to Ritual Offerings (Nephillim Press), and a few other places on blogs and forums. I will briefly summarize here what I had to say on the subject elsewhere: