Princess Twilight Sparkle, is not only Princess Celestia's faithful student and the Element of Magic, she is also the top ranked of Celestia's elite government assassins, The Justiciars, hunting down any criminals that have escaped justice. Their motto:

KILLER IS DEAD

(I need cover art, but I don't have money, so, any pictures somepony can find are appreciated, and I need the URL aswell, thanks! Comments appreciated!)