> In hindsight, it was definitely the concept of original sin that got my young mind spinning, not understanding how for example a newborn baby could be innately sinful.

I have no qualms believing that. Watching a young child empirically discover the theory of mind, and then progress to deceit without being taught is almost miraculous. Humans are inherently selfish and short-sighted, a huge part of parenting is spent on tempering these instincts.