actually the Inquisitors are above the law as Inquisitor Eisenhorn states in the beginning when he is accused of the crime of killing an entire worlds noble population. He mentions to the reader that he could just leave that there laws had no power over him.
The Inquisition is the law. They have the power to wipe out an entire planet if the need is dire enough. There is no second guessing them except by a trial of there peers or higher ups. They only ask the Adeptus Astartes for help instead of demanding it simply because of the great service they have done for the emperor
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