A regional term in the South and parts of the Southwest. A person can become such a trial and burden on those around hin that when someone finally shoots him, everyone who knew him says, "Well... He needed killing." It was often used in reference to outlaws when word arrived of their deaths.
In modern times the usage is more in terms of referring to a bad sort as "he needs killing". There are, however, several recorded instances where someone on trial for murder has attempted to use it as a defense - sort of a 'self-defense after long history of intolerable actions' kind of thing. Sometimes the dirty, rotten *&%#@%&#* NEEDED KILLING!
When a notorious criminal is executed: "He needed killing!"