Lawful evil is a tendency in RPG alignments that denotes a lawful behavior (ethics) in the chaotic-lawful axis and evil morality in the good-evil axis.
A lawful evil dictator that rules with an iron fist and oppresses its people is an example of lawful evil creature.
A character in Dungeons & Dragons games with a "Lawful Good" alignment, played poorly and to the detriment of the game, usually resulting in the deaths of the character and his allies.
(See also: Chaotic Stupid)
Lawful Stupid Player: My Paladin detects evil on the wizard, and attacks!
Other Players: But he's got an ogre and ten orcs...