What is most natural of all is the self-replication of behavior from one generation to the next. Children have always tended to emulate their parents and always will. Obviously, there are many individual exceptions to this tendency, but very rare exceptions at the group generality level. While aphorisms and 'common sense' should tell us that what we model for our children is what they will copy, actual science is also helpful because 'common sense' tells many people that smacking their children is how the children learn respect, learn to mind, learn to behave. Science can help us parse common sense from everyday foolishness.
Social norms can change faster than behavior--e.g., social norms have changed dramatically, thankfully, since my childhood, on spousal violence, but the social practice of wife-beating is not yet ended, sadly. The reverse is also true. When the law changed in many places on domestic violence, the social norms of the police officers who enforced the new and better laws often lagged far behind. So which comes first, new behavior or new norms, new laws or new practices? Clearly, that depends.
The easiest shift from childhood cultural influences, of course, is immersion in another culture. While some individuals resist changing to adapt and emulate new cultural values and practices, many follow the new herd in which they find themselves. As an inmate, I've seen this happen to those who "didn't belong in jail" in the sense that clearly, their values were not typical of the average inmate upon arrival, but I've seen them immersed in a new "moral Maytag" for even a week and shift behavior downwards in response. Soon they regard women, people of different ethic groups, or anyone not adequately represented in the inmate population as objectified others and the pejorative labels appear, used until challenged (which, as an older inmate with less malleable characteristics, was my self-appointed role).
This also applies in reverse. Most people suddenly immersed in a culture of better (more just, more accepting, less violent) values will usually begin to manifest at least verbal, then behavioral, agreement with those values (though they may retain plenty of vestigial inner doubts and objections). This is why successful campaign for social change argue not for a change in values, but for better cultural enforcement of the values you want them to believe they already hold. Of course we are all for peace, and here is one way we can help it arrive more quickly. Naturally we want a culture that is friendly and open and fair to all, and here is one thing we can do toward our common goal.
Nature or nurture? Yes. We are in a dialectical relationship with our own DNA, we create self-replicating generations and self-fulfilling prophesies all the time. Intervening in this is difficult and required if we want to shift our culture to peace, justice, and nonviolence. We can nurture our new nature but that takes courage and discipline. Fortunately, we can also nurture our natural courage and discipline until it produces the world we want.
Fry, D. P. (2005). The human potential for peace: An anthropological challenge to assumptions about war and violence. New York, NY: Oxford University Press.