Treachery is the sin of acting against people to whom one has an obligation - for example family members, friends or one's country (in the last case, it is called treason). In most cultures, a treacherous act is considered far worse than an equivalent crime against someone with whom one has no connection at all, although Jesus often emphasised the importance of doing good to everyone and not just one's own people or family (e.g. Matthew 5:46-48).
Famous examples of treachery include Judas Iscariot's betrayal of Jesus, Brutus and Cassius' plot against Julius Caesar, and Benedict Arnold's actions during the American War of Independence. In Christian tradition, the first traitor was Satan, who led a rebellion against God and was cast out of Heaven as punishment.
In Dante's Inferno, treachery is considered the worst of all sins, and traitors are frozen in ice in the lowest circle of Hell.
A modern example of treachery, in this case treason is Bradley Manning's betrayal of the USA in handing over documents to Wikileaks.