It could also be argued that the socialization process is integral to brain development and without it, behavior coded by DNA would never appear.
That's true too. "Nature through nurture" replaces "nature or nurture".
If you have more evidence that humans are good by nature and not by social forces, I would be interested to see it.
Among the items on Donald Brown's list of anthropological characteristics for which there are no known exceptions across all societies:
actions under self-control distinguished from those not under control
distinguishing right and wrong
economic inequalities
economic inequalities, consciousness of
fairness (equity), concept of
food sharing
generosity admired
gift giving
healing the sick (or attempting to)
hospitality
incest between mother and son unthinkable or tabooed
inheritance rules
judging others
law (rights and obligations)
law (rules of membership)
males more prone to theft
moral sentiments
moral sentiments, limited effective range of
murder proscribed
promise
property
rape proscribed
reciprocal exchanges (0f labor, goods, or services)
reciprocity, negative (revenge, retaliation)
redress of wrongs
resistance to abuse of power, to dominance
sanctions for crimes against the collectivity
sanctions include removal from the social unit
semantic category of giving
sexual modesty
sexual regulation
sexual regulation includes incest prevention
shame
stinginess, disapproval of
taboos
tabooed foods
tabooed utterances
territoriality
trade
turn-taking
violence, some forms of proscribed
If morality were purely cultural, for most of the stuff on that list wouldn't there be a society somewhere that didn't go in for it?
Also, lots of other primates display moral behavior. It's well documented in
"Good Natured", by Frans de Waal. To suppose that a moral sense is not grown but only learned is to suppose that lemurs and monkeys and apes evolved progressively more sophisticated moral instincts, and then hominid ancestors lost them, and then all human cultures reinvented them by social evolution. It's unparsimonious.
Also, some aspects of human morality don't make any sense from an individual perspective. So if a culture tried to teach them it's not clear how it could motivate its members. Punishment, for example. It's in your best interests to leave the dangerous policing of other people's morals to somebody else. Yet people experience a powerful emotional drive to hurt those who break the rules, even when it gets them in a fight they might lose. That only makes sense from a genetic perspective -- when the genes for morality aren't uniform through the whole population, the urge to punish wrongdoers is often the suppression of competing alleles to your own. So if you get hurt beating up a bad guy the gene that makes you do it is in your cousins too, so it still wins.
It is a tempting idea, in that it does relieve us of some responsibility for our behavior, which some people might find comforting.
The idea that it's purely cultural can be equally tempting in the same way. "It's not my fault that I'm so evil. It's society, society." If we decide identifying causes relieves us of responsibility, a cause is a cause.