Neutral Evil characters are perhaps the most simple of all the alignments to run or play. They are concerned only with themselves and what pleases themselves. Not overly concerned with the ethical concerns of Law and Chaos, they act in accordance to what best suits them and all other concerns are left alone. Neutral Evil characters have a tendency to be more isolated than other alignments. Unable to be trusted, they in turn give no trust to anyone. They always have a double cross ready, but will usually only use one to betray someone who has betrayed them. It is not that they have any moral compulsions about betrayal, Neutral Evil characters recognize that the less you betray others the less likely you will be betrayed. When Neutral Evil characters do betray, they usually kill the betrayed to reduce complications down the road.
Unlike Lawful Evil characters, who view others as tools to be used or Chaotic Evil characters, who see others as persons to bully into submission, Neutral Evil characters view other persons as "complications." Whether or not that person is a desirable complication (or at least tolerable) determines the length of the relationship between the two. The relative power of the characters often determines what the Neutral Evil character will do. Neutral Evil characters can be controlled (somewhat) via the threat of retribution, but they will do what they can get away with. If that is anything then may the gods have mercy on anyone in their path that they take a disliking to.
Neutral Evil societies are usually dominated by a dictator strong enough to keep the underlings in line. When the ruler loses strength he usually loses his life. Neutral Evil societies are rarely long lasting or very large since the members of that alignment rarely cooperate and tend to disperse in order to take advantage of those more foolish than themselves.