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A lawful evil villain methodically takes what they want within the limits of their code of conduct without regard for whom it hurts. They care about tradition, loyalty, and order but not about freedom, dignity, or life. They play by the rules but without mercy or compassion. They're comfortable in a hierarchy and would like to rule, but is willing to serve. They condemn others not according to their actions but according to race, religion, homeland, or social rank. They loath to break laws or promises.
This reluctance comes partly from their nature and partly because they depend on order to protect themselves from those who oppose them on moral grounds. Some lawful evil villains have particular taboos, such as not killing in cold blood (but having underlings do it) or not letting children come to harm (if it can be helped). They imagine that these compunctions put them above unprincipled villains.
Some lawful evil people and creatures commit themselves to evil with a zeal like that of a crusader committed to good. Beyond being willing to hurt others for their own ends, they take pleasure in spreading evil as an end unto itself. They may also see doing evil as part of a duty to an evil deity or master.
Lawful evil is sometimes called “diabolical,” because devils are the epitome of lawful evil.
Lawful evil is the most dangerous alignment because it represents methodical, intentional, and frequently successful evil.