Good and evil are ideas that humans made up. They are subjective and because there is no such thing as universal morality. There is only a consensus that a bunch of humans have generally agreed to, with variations depending on your human cluster of choice. Humans are flawed and so shouldn't really be the basis for a moral code, much less a bunch of humans.
If we're using the general consensus as the measuring stick, you'd have to wade through all of the scientific papers relating to nature or nurture. A lot of which are conflicting. If any consensus can be derived from those and rule out any outliers, it is that humans are born with certain predispositions and character traits just like they are born with certain physical traits they grow into. The environment they experience as they develop does effect the way some of these traits develop and can, in the end produce a wide variety of results.
tldr: they can be both