I still subscribe to the blank slate theory. The problem is that the world is so full of insanity and evil that they corrupt very quickly without proper guidance.
My take on it is that youth corrupt with no outside influence. That doesn’t mean that they have evil intent. It just means that like you said, they need to have adult supervision until they grow old enough to understand right from wrong.
They do have a blank slate, but even natural curiosity presents them with so many poor choices, that without adult supervision, children would grow up to be quite evil indeed.
It would be evil born of innocence, but evil none the less.
You don’t need to be exposed to the world at large, to be selfish, to covet the possessions of others. It’s a natural instinct, and one that needs course correction along the way. This is only one example of a natural urge that needs to be molded into something healthy.
This is only my take on it, so take it for what it is.