It is noted that to perform the Unforgivable Curses, the caster must "mean it". This means that they need to want the effects a fair amount for the effects to last. In Order of the Phoenix, Harry attempts to use the Cruciatus Curse on Bellatrix, but he doesn't 'mean it', as Bellatrix later instructs him to, and the effects are temporary, causing her to only feel the pain for a moment.
The use of the Unforgivable Curses has existed since the time before the light and was authorized against Voldemort and his followers by Bartemius Crouch Sr, during the First Wizarding War. Although not stated, it is possible that this was limited to the Imperius Curse (to force a Death Eater to submit to questioning or surrender) and to the Killing Curse (as a last resort). Shortly after his resurrection, Voldemort names two Death Eaters "killed by Aurors".
The Unforgivable Curses:
Avada Kedavra (the Killing Curse)
Crucio (the Cruciatus Curse)
Imperio (the Imperius Curse)