As far as I am concerned, good and evil are concepts arising naturally through social interaction within a community as a way for that community to regulate and streamline internal processes and to avoid things that are deemed of negative effect to the community. Humans, as social beings have a natural tendency to form ideas about good and bad, based upon what they gather from the community, coupled with their own deductive reasoning. Psychopaths are an example of what happens when these systems fail.
This means that good and evil as abstract concepts are inherent to humanity, but what form they take is entirely dependent upon the community one grows up in and even the treatment of good and evil can differ between communities, with some holding a strict totalitarian view in which good is absolute and evil needs to be abolished, while others see it more as a balancing act.
If we take a look at the nature of good and evil itself and the treatment of them within society, Nietzche's philosophy clearly states that to do good one sometimes has to be prepared to do evil. One thing interesting is that this is at least partially backed up by
Game theory. A community is most stable and healthy if most of it's members adhere to the rules, the so called sheep. But a society of only sheep is stagnant and will quickly be overtaken by societies that are less rigid and give some space to the so-called wolves, actors who are less concerned with adhering to the rules at all times and instead tend to draw their own path. The aforementioned "church of Satan" poses itself as an ideology aimed specifically at "wolves" and many of it's ideas (save for the occult ones) can be taken as a pretty coarse yet decent description of what a "wolf" actually is. A society where there are too many wolves would however quickly devolve into a kleptocracy and become entirely unworkable.
The distribution of sheep and wolves are of course dependent upon the society in question, but the reverse is also true. Societies are generally not in a stable equilibrium, if they are in equilibrium at all. Instead they kind of evolve. In this case it is useful to describe societies in the form of a collection of
memes, essentially the bits of a societies ideas. Memes are similarly to genes in that they can appear, disappear and change depending upon external and internal factors. The entire system of memes that make up a society is probably (note that this is completely my own point of view) a chaotic system. Normally it will fluctuate within a set of ill-defined boundaries (for example the memes related to promiscuity and sex, to which the stance within western society became very open-minded in the '60 only to fling back to a more conservative yet still obsessed point of view later on). It will react to outside influences which causes these boundaries and the baseline to shift. If the shift is too rapid so that the memes can't catch up you get a partial or complete breakdown of the system.
A good example of partial breakdown would probably be the rise of Fascism all across the western and middle eastern world in the late '20s and early '30s. On the other hand a radical change applied slowly enough does not cause a breakdown but instead lets the system adapt. The rise of information technology is an example of this. It strongly influences large parts of society yet it was and still is gradual enough to allow most people to catch up. Interestingly enough some can't catch up and as a result it is possible that for some people, especial the illiterate and the elderly, a breakdown occurs without the entire society suffering from it. Another example of this would be the switch from national currencies to the Euro within the EU, which overall was adapted to reasonably well, but caused a bit of a breakdown for some who could not adapt.
Bringing this all together, we see that good and evil are external, independent of society, but what good and evil actually entail is defined entirely by society. They are, like the society that defines them, constantly changing as a result of outside influences and the general instability of societies themselves.