In thinking about such scenarios, people seem less unanimous than you'd imagine they might be, that what would happen would be: nearly everyone would let rip, and do as their impulses dictated -- rapine, robbery and mass-murder to the max. I seem to remember a thing from Neville Shute's novel On The Beach -- premise, the Cold War goes nuclear, and all life on earth is being wiped out by a deadly radioactive cloud; which because of climate-type factors, gets to Australia last of all. If I recall correctly, the author surprises his readership re the Australian end of things in this situation: by (while there's a certain degree of riot and mayhem) people in the main, just stolidly going on obeying the laws, doing their jobs, and carrying on as normal, until the lethal stuff arrives and bumps them all off. Though -- it might be significant that the action of On The Beach is set in Melbourne; which I gather has a reputation for being the most boringly goody-goody city in Australia. Maybe Shute had a point to make about Melbourne here?