To wound the innocence in another is to teach oneself that innocence can be wounded, and no lesson costs the mind more. The one who attacks the little one attacks the belief that anyone is safe, and having taught guilt he must now believe in it. This is the millstone: not a punishment sent from outside, but the sheer weight of a mind convinced that it has made another guilty, sinking under a burden it laid upon itself.
The warning is mercy, not threat. It shows how heavy attack becomes for the one who chooses it, so that a mind might set the stone down before it drowns. What is loosed in forgiveness was never anything but this weight, released the moment the other is seen as innocent still.
Little ones: the untouched innocence in every mind, the truth the ego most wants to convince you can be harmed
Millstone: the crushing weight of believing one has made another guilty, the burden attack hangs upon its own thought
Depth of the sea: the sinking of a mind into the darkness of its own condemnation, where guilt pulls it under