You're entirely on the right track.
Common behavior in games like DayZ fairly accurately mirrors behavior that is appropriate in a situation where the rule of law has entirely disappeared.
Just look at Maslow's hierarchy of needs and it should become far more clear. Morality, at least as we generally conceive it, is something that doesn't really come up until pretty much every other need is fulfilled. In a situation without the rule of law, individuals are generally forced to spend much of their time working in the bottom two levels of the pyramid.
Thus, the logical reaction to new persons revolves around our needs and the available resources. Since there is little benefit to be gained (at least in DayZ) from working with strangers, and a huge amount of risk, there are really only two options: Kill them, or avoid them. Since avoiding other people is often so difficult, and they have little to offer you other than your death, this leads to people defaulting to the other option.
As the benefits of cooperation increase, and the penalty for dying goes down (read: when you can securely cache excess equipment for use with future characters), I suspect that we'll see less KOS, more avoidance, and more cooperation.