Why did ancient pagans practice blood sacrifices?
Ancient pagans practiced blood sacrifice for the same reason that ancient Jews and Christians did. (And, oh yes, they did. Read the Old Testament…animal sacrifice is mentioned multiple times.) Blood is considered part of sympathetic magic in that it, more than any other thing, can ritualistically represent the life of a specific being. It is also considered precious, as it is necessary for life. Thus, it was the most precious sacrifice that could be made.