If core moral principles occur through all societies that argues against such a position. Otherwise one would expect great variations in it. Additional moral analogues occur in pre-Society creatures for want of a better term. We may phrase our values in terms of morality yet those same values exist objectively beyond just those creatures that can be said to have a society (animal kingdom is rife with our most cherished values). Some "morality" is froth on the surface and varies all the time - sexual mores chief amongst them - but the OP is concerned with "Evil". And that suggests to me core moral principles that are nearly universal.
So, I should clarify that I think it's probably a mix of both, the point I was making, and the issue I was taking, was with Dawkin's approach to morality which I find quite black and white. I'm willing to concede that the ingrained core tenets of our moral code which effectively boil down to not doing harm to others except within certain circumstances, are probably programmed as a survival mechanism. However I think what we see in modern society is a widening of the net in terms of those pre-existing moral codes to include those groups that would have been considered outsiders previously, homosexuals, other ethnicities, animals, and also a narrowing of the acceptable reasons for stepping outside of the moral norms. Now you're right that these fluctuate but I'd argue that the prevailing direction is widening of inclusion and narrowing of exceptions.
One of the difficulties in discussing evil is fact that the term is so ingrained in religious dogma. A few hundred years ago homosexuals were evil, centuries before that women who stepped out of line were evil, before that blasphemy was associated with evil. I suppose really what we're asking is, are there people out there who are predisposed towards being incapable of operating in a way that society will deem to be, on balance, morally acceptable, and is intelligence a pre-requisite to this?
Question: Would you include upbringing in that such that a person didn't believe what they did was wrong due to their beliefs? Your "not knowing" principle would seem to support this as well as insanity. And if you do extend it to upbringing then what prevents you from extending it to a society or social group?
It's when you start applying morality to real life situations that the application of morality itself becomes a tricky moral quandary, because you have to decide where you draw the line, and that has consequences. In answer your question, maybe. For me belief is only half of the equation, there is also understanding. Take somebody like Anders Brevik for example, I have no doubt he believed that what he was doing was right, even though the rest of the World was appalled by his actions, but he was also quite aware that society would not see it this way, and he was aware of the law and the reasons for the law, so in cases like that then no, I wouldn't extend the same understanding.
However you could also look at the examples of, for instance, a lost tribe who would kill outsiders, either due to fear, a custom of isolation, tradition, whatever. In that example I would say the tribe members aren't able to adhere to the morality of wider society as they haven't had any exposure to it, in their culture what they did was right and they have no reason or opportunity to challenge that view.
I suppose in between the two is the conditioning one might receive through upbringing, but again that's accounted for in law where somebody who is subject to such a level of undue influence from a family member or authority figure or group that they effectively cease to be responsible for their own actions. For instance you could have somebody who has had a crap upbringing, who has only ever known abuse but who fundamentally understands that those actions are wrong, but then you get the people who have the same situation growing up, and on top of that they're conditioned from birth to believe, truly believe, that the state and wider society are Evil constructs out to persecute and hurt their families.
It's a tricky one but basically, as with anything the line eventually gets drawn through experience, how much undue influence is required before a person's free will is eroded and they've effectively become conditioned through grooming?
