(This text was written specificaly for D&D, and may thus need alterations.)
Lawful evil characters use a society's structure and laws to their own advantage. They will play by the rules without mercy or compassion, to take what they want without regard for whim it hurts. They care about tradition, loyalty and order; but not of freedom, dignity or life. They are comfortable in a hierarchy and enjoy ruling, but are willing to serve out of fear of punishment. They will condemn others not according to their actions, but according to their race, religion, homeland or social rank.
However, because they depend on order and law to protect themselves against those who oppose them on moral grounds, they will almost always honour a lawful oath or contract that they have made, even if it turns out to be unfavourable for them. Because of this, Lawful Evil characters tend to be very careful when giving out their word.
Lawful evil characters may have particular taboos, such as not killing cold blood or not letting children come into harm; and like to imagine that these compunctions put them above unprincipled villains. Other Lawful evil characters will commit themselves to evil with a zeal like that of a crusader committed to good.