In the mythology of DF, it seems that there are some solid concepts "good" and "evil". This differs from the real world; in our universe, (at least from a secular perspective), these concepts are much more vague.
One might say that in the real world, the concepts are "what is good for the group" and "what is good for the one". All religions of any measure stress community and altruism, and it is true that such "good" behavior may not improve the survival of the individual, but certainly that of the group as whole.
Now here's where this gets interesting; is it not true that Goblins (and Kobolds and Ratmen and Frogmen and Elves even), in siege-ing your fortress, show self-sacrifice for the good of the whole group? Consider, your fortress suddenly appears deep into new territory, builds up huge supplies of goods and an army, and in future versions will be sending raiding parties out- a clear threat! Goblins take children for the benifit of the Dark Tower and to demoralize the enemy, and so on.
The fact is, an "evil" culture cannot survive- not because of outside influence by a "good" force, but simply from self-destruction.
What is needed is a structure that means that the chaotic structure of a goblin fortress is stable, and that will emergently produce the kind of action the goblins are famous for. In other words, we have to make a "good" that is Evil.
The Goblin's definition of Good- as in "Good for the group"- has to be something that:
-Is functional.
-Survives and encourages internal strife.
-Leads to invasions of neighbors.
-Is lead, at least in part, by Demons.
This means that an evil goblin's social structure will be:
-Without internal strife
-Promotes interculturaism
-Denies Demons, an athiest
I don't know what would work best to do this, but let's see what we can come up with.
I would say, most likely goblins reproduce rapidly- to counteract the rapid loss of goblins. Families do not exist- pairings are rarely mutual and even more rarely long-term. Infants are raised by the mother for a few months or years, then run in clannish gangs of which there are many in a set of dark towers.
These gangs are not necessarily related by blood, but they are tight-knit socially; if a goblin gets into a position of power, he will help his clan above others. This leads to internal strife as the various clans struggle for domination of the power structure. Presumably, assassins are always at work within the upper echelons. Indeed, for a goblin, it may be safer to be in the military, where one is respected and supported but without the authority or power to choose when and why to go to war, only how.
This constantly roiling power structure ensures that only the most subtle and skilled legislators can get power in the social order. The power structure, then, is the priestly and bureaucratic class or caste that is constantly assassinating competitors, superiors, successors, rival clan members, and bystanders; leaving only the most fit- strong, clever, or well-spoken enough to avoid assassination. The various leaders of this order decide whom to go to war against, where to build new towers, and who must be killed today. They do this at the vague direction of The Demon, who is the constant of the goblin social structure; always there, distantly directing the goblin followers via the byzantine Bureaucratic class. Most day-to-day and perhaps year-to-year management is handled by the bureaucrats, but the Demon holds the splintering structure together to prevent clans from annihilating each other and slowly directs the whole structure against whomever it is that is next on the Demon's list of targets.
The Evil goblin, of course, would be one that lets a weak leader remain, who abandons his clan once he comes to power, who trusts an outsider without obvious gain, and who ignores the will of the Demon.
Of course, this is all a pointless exercise. Toady One may post here stating that goblins have Evil DNA and that's that or something. My point is that the goblin structure, and for that matter any social structure, if developed well, will add to gameplay immensely by changing how you respond to the situations. Imagine that as a goblin overlord, you have a noble who wants a Blue Diamond Bed- and you only have to order him assassinated and one of the lower classes will come along and whack him. Wouldn't that be GREAT?