What have you read from modern liberals that shows they have selectively ignored that fact?
Take this website for instance:
http://www.britainexpress.com/History/Celtic_Britain.htm
It completely glosses over their culture of human sacrifice and dismisses it as basically roman propaganda, despite archeological evidence such as the Lindow Bog Man and the discover of a mass grave of headless skeletons in Alveston, amongst other things, supporting Roman sources...