They were old towns for the most parts, and ones that had been through a number of bushfires in the past, just never one as intense as this. That's why the death toll was so high, usually you are safe staying inside your house, even if it catches fire it will burn slowly enough that you will be able to get out easily after the fire has moved on. You may lose the house after that, but you will be alive.

This time the fire was so incredibly fast and hot that the houses were blazing while the fire was burning hot all around. Strategies that had always worked before no longer did.

And I agree with you that it is not possible to understand the mind of the arsonists (there were several fires lit in different parts of the state). It was the hottest day on record, with gale-force winds. They must have known that death would be the result. At least one of the arsonists is in custody, investigations are closing in on others.