On the surface, it seems a little surreal to state that there was "nothing aggressive" about a mass murderer. After all, murder - especially a spree killing in a school - tend to be quite an agressive act. Your Thomas Hamiltons and Michael Ryans and known for their passive behaviour. But then again, it is only their final acts that appear so aggressive and so violent.

Tim Kretschmer probably wasn't aggressive before he went on his shooting spree. He was probably a very normal looking young man. There were probably very few warning signs that he was about to perpetrate a terrible act of violence. And that is why all those who call for the authorities to recognise the warning signs are on a hiding to nothing. The warning signs won't be there - Kretschmer won't have sat around muttering "kill 'em all, kill 'em all - gonna fucking kill 'em all." Any signs of his burgeoning rage could probably have been interpreted as signs of being a teenager.

Part of the tragedy of these mass killings is that they are arbitrary and they are random. You can't predict them; you can't stop them unless you imprison everyone who ever appears to be slightly weird. It is comforting in the aftermath to talk about identifying warning signs in the future. The reality is somewhat different, and behind that reality is the unpleasant truth that you probably can't stop these things from happening.