Lawful Evil is one of the Nine Alignments, combining Lawfulnes with "Evil" (malevolent intentions, being oriented towards the Dark Side, etc.), both a game mechanic, and an actual concept within the universe of the franchise itself.
Lawful Evil characters have no delusions about following morals, look down on concepts like kindness, humility and altruism, as they are primarily concerned with themselves and their own little agendas. Throughout their actions, they pay lip-service to traditions, loyalties and order, but they care not for freedom, life or dignity. They play by the rules, but they show absolutely no pity or mercy.
They observe law and tradition religiously, but not out of moral obligation, but because they believe that this protects their reputation, helps them advance their agenda in secret, and undermines more reckless competition. They seldom make promises, but when they do, they intend to keep them, as they have a repuation to keep up. They love hierarchies, but always strive to be at the top of them: they resent lack of upward social mobility when they're on the bottom, but do everything in their power to abolish said social mobility once they reach the top. They abhor getting into open confrontation with the very same laws that protect them, don't like getting their hands dirty (as they have a reputation to keep up), and often go one step further by creating personal taboos for themselves - taboos that obviously don't apply to their minions and hirelings, such as not being allowed to kill in cold blood or not being allowed to kill children. For the Lawful Evil character, openy going against the law in public and getting their hands dirty in front of the prying eyes of the law is an absolutely last resort, and they always look for alternatives, if there are any.
They may be evil, not afraid of using morally abhorent methods to achieve the goals (or just satisfy their own desires), but they consider the law to be their friend, a tool to be exploited, a shield against reckless competition and a weapon against law-breaking do-gooders. The most succesuful vilains tend to be Lawful Evil, because they can remain hidden in plains sight, advancing their agenda in secret, fooling everyone into mistaking him/her for a reasonable, even good-intentioned figure.
The Lawful Evil alignment is very common amonst vampires (especially ones that work in groups and observe some kind of group tradition), selfish aristocrats and merchants, necromancers and death knights who live in Keldorn.