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"Because the Wolves kill cleanly, and we do not. They also kill quickly, and we have never done that, either. They fight, they win, and they stalk back to their ships with their tails held high. If they were ever ordered to destroy another Legion, they would do it by hurling warrior against warrior, seeking to grind their enemies down with the admirable delusions of the 'noble savage'. If we were ever ordered to assault another Legion, we would virus bomb their recruitment worlds; slaughter their serfs and slaves; poison their gene-seed repositories and spend the next dozen decades watching them die slow, humiliating deaths. Night after night, raid after raid, we'd overwhelm stragglers from their fleets and bleach their skulls to hang from our armour, until none remained. But that isn't the quick execution the Emperor needs, is it?
The Wolves go for the throat. We go for the eyes. Then the tongue. Then the hands. Then the feet. Then we skin the crippled remains, and offer it up as an example to any still bearing witness. The wolves were warriors before they became soldiers. We were murderers first, last, and always."
-- First Captain Sevatar, when asked why the Night Lords aren't the Emperor's sanction force against other Legions.