The Romans, esp Caesar, accused all their enemies of, being large, powerful, conducting human sacrifice, and generally being scumbag barbarians. So I wouldn't put to much faith in Roman history as it was mostly an excuse to attack, wreak and enslave. Always thought it was funny that the Romans accused others of human sacrifice while they had thousands torn to pieces in amphitheatre for entertainment all over the empire.

I’ve always wondered if the Druid roots come from the religious practices and beliefs of the original hunter gatherers that pre date the celts. They had knowledge of astronomy as evidenced by some of the ancient monuments that work with solar and astrological events. The Celts are now believed to have migrated and blended with natives rather than conquering. It stands to reason there was some cultural exchange. It would explain the mystery around the Druids. Why it came from Britain. We have little explanation for the ancient buildings here and in Ireland that predate the pyramids and Celts. I think it’s a reasonable assumption anyway. That Celtic beliefs blended with the original inhabitants, those that are still represented in British and Irish DNA.