Machiavelli, Discourses I.42:
It should be noted, too, in the the affair of the Decemviri how easily men are corrupted, and in nature become transformed, however good they may be, and however well taught. Consider, for instance, how the young men whom Appius chose as a bodyguard, soon became the friends of tyranny for the sake of the small advantages which accrued; and how Quintus Fabius, one of the second Ten, though an excellent fellow, was after a while blinded by a little ambition and, under the evil influence of Appius, changed his good habits for bad and became like him.
Due consideration of this will cause all legislators, whether in a republic or a kingdom, to be all the more ready to restrain human appetites and to deprive them of all hope of doing wrong with impunity.