I was lunching with a friend this week when he said something that made me start.

We were talking about the terrible case of the little girl thrown over the Westgate Bridge in Melbourne the week before. It is something that particularly and deeply horrified us.

"Of course," my friend said, "one of the reasons is it's something we could imagine ourselves doing, just for a moment".

I started because I thought he was right.

The thing that is most terrifying about that experience is that anyone who has had children knows what it is like to be angry and just for a second - a momentary second - could imagine doing something as horrific and cruel as allegedly occurred in this case.

There are many crimes and terrible things I can't imagine myself doing, and in some sense they therefore appear distant and abstractly terrible.

But it's when you can recognise in yourself just the seed of that which in another has been so terribly expressed that you truly feel the alarm and horror.

It means that our denunciation of such acts is not just on behalf of the innocent victim but also as a warning to ourselves about those parts of darkness that lurk in all of us.