Why do murder and stealing between humans happen? Or, more importantly, why do we have laws that discourage murder and stealing if these are predicted by natural selection?
I'll answer the question in two ways: first, murder is a human behavior in which there is a deliberate killing of another human, usually for different and changeable reasons. (There are varying degrees of killing but I don't see the need to itemize them here, suffice to say that murder is a 'bad' thing.) If one were to assess murder from an evolutionary perspective a number of explanations can arise. For instance, the environment herself can stress individuals and their species groups to the point that a necessary culling of fellow species begins, the sexual selection process and retention of mating partners can sometimes result in the death of species members, control of group or a change in hierarchy sometimes results in the death of leaders or challengers to that position, along with many other instances as well. In other species these reasons are justified in the struggle for survival that each faces every day. For thousands of years before civilization humans would have struggled with such instincts in their small species groups as well.
But all this is fine until we look at the world outside. What we see is not the random killing and the strong oppressing the weaker. Rather we observe tightly knit species groups that prefer to protect than to injure or kill each other. From ants through piranhas to humans, all species tend to cooperate rather than kill each other, the question is why? Do they all adhere to a moral law supplied by an almighty creator or is there a natural explanation for all this? I suggest the best explanation is natural selection. Today, we do not observe human societies that instead of cooperation with each other preferred to kill, steal, rape, and lie to each other. These societies, if they ever existed, are dead. The Homo sapien has been around on this planet for perhaps a million years and before us our ancestors as well, we have had plenty of time to learn and fluctuate between practices and behaviors in that time period. Today most functioning societies have some sort of 'law' against anti-social behavior such as murder and stealing, not because it was given to them by a god, but because without it there would be no society. There is no tautological reasoning here, instead this is directly observed in species-level group interaction across the board regarding social animals. Even solitary animals such as cats will not fight to the death with a rival.
I'm not entirely sure what you mean when you say "if humans were left to their own devices' as if this is somehow a foreboding and undesirable outcome. On the contrary, if humans are left to their own devices they invariably form social groups that rely on unspoken ideals and laws about not killing or stealing. These laws are not unspoken because they perhaps exist ‘within’ us but because the very functioning of a social group will not occur if such good behavior is rejected for anti-social. Let me ask you a question, if your god were to come down to earth now and tell everybody on the planet that he was leaving for good and never coming back, could humans still be moral beings? When you answer this it should become clear that morals do not come from god.