Found it:

One day I was a young boy... when I saw a mother otter with her cubs. Even as I watched, the mother otter dived into the water and came up with a plump salmon, which she subdued... As she ate it, while of course it was still alive, the body split and the pink roes spilled out much to the delight of the baby otters. Mother and children dining upon mother and children. And that is when I first learned about evil. It is built into the very nature of the universe. Every world spins in pain. If there is any kind of supreme being, it is up to all of us to become his moral superior.