One thing I often come across in researching historical pagan religions is the frequent use of animal, and occasionally human, sacrifices. For instance, sacrifice appears to have been the primary method of worship during a large part of ancient Greek history. To give a specific example, Hecate was strongly associated with dogs and dogs were occasionally sacrificed to her. Within Germanic paganism, human and animal sacrifices seem to have been a fairly frequent practice. Accounts of the major temple that existed in Uppsala in modern day Sweden were particularly gruesome as the sacrifices would be left to hang and rot in the trees surrounding the temple such that the trees around the area were completely covered in the rotting corpses of animals and, potentially, humans. Of the Celts it is hard to say for sure what many of their specific beliefs and practices were since much of what we know about Celtic practices, particularly in continental Europe, is considered rather dubious since much of it was written by the Romans and others who were often at war with the Celts and portrayed them in a negative light for propaganda purposes. Still, it's almost certain that druids officiated over some sort of sacrifices, though scholars seem torn about whether or not this included humans.
This is one aspect of paganism that I have a little bit of a hard time figuring out. Obviously, the vast majority of pagans in the modern world do not engage in any kind of murder or animal cruelty, and those that do are usually found to be mentally ill and drawn to these practices for that reason rather than for any particular religious reasons. What I'm curious about is how modern pagans reconcile their current religious practices with historical evidence of human and animal sacrifices, though.
Quite easily in my case. There is some evidence indicating that the people sacrificed in the past that were related to or touched upon my path in some way were treated as royalty beforehand and may have been if not volunteers then at least willing. But just as much lore and information about Druids and ancient Celtic religions are derived from educated speculation based on archeology etc my path is a *very* loose reconstruction thereof and as such is tailored to a modern life. Therefore blood sacrifices are not only not required but forbidden in most cases. I can take the lessons from the past and apply them to my current life and worship without having to embrace any type of blood sacrifice. There's nothing to reconcile.