Fire It Up!

I had never really given much thought to the topic, and it is seemingly obvious, but last night I saw a picture of some rioters throwing petrol bombs and I was struck by how instinctive is man's desire to set fire to things.

Of course this leads on to the usual trite observations about the destruction, ritualism and purification of fire, but I'm more interested in the behavioral side of things - why do people in this modern world, with just about any weapon they could want available to them, (most of which are far more efficient) still automatically want to set fire to things, if given the chance (or, more precisely, when involved in mob behavior) - and at what point did this particular instinct become hardwired into our DNA?
Evidence for controlled use of fire has been proposed for burnt bones discovered in South Africa, dated at 1.5 million years old; so we have certainly had plenty of time to allow fire into our genetic code

Was there a time when our ancestors habitually set fire to the grassland resources of competing tribes, in order to remove their energy source, either from spite, or to neutralise the men, leaving the women free to be easily abducted and added to the stronger tribe? This behaviour has certainly been observed being carried out by tribesmen in recent history.

Perhaps we used fire to destroy the unfortunate Neanderthals