I found this take from Mel about the creepy Satan baby he put in The Passion. I think he might have some other kind of symbolism in mind but I'm not sure what it is.

I thought the relational connection between Jesus and his Mother was the most interesting and compelling part of the Passion. Now that I have my own children I writhe at the thought of my child suffering at all. The pivotal scene in the movie for me was when Mary tries to soothe Jesus as he carries the cross and says, "I'm here son." And he replies, "See Mother, how I make all things new." This statement struck me like a hammer. Changing this broken world for the better comes from suffering and sacrifice; and Jesus has come not to make all new things but to actually renew the world to surpass it's original glorious nature. These are uniquely Christian concepts that ring out powerfully in this scene. This alone was worth the price of admission for me.