I am presently reading Enemies: A History of the FBI, and encountered this insightful comment by Associate Justice Louis Brandeis, in the Olmstead vs. US (1928) decision.
“The greatest dangers to liberty lie in insidious encroachments by men of zeal, well-meaning but without understanding. [….] Crime is contagious. If the government becomes a law-breaker, it breeds contempt for the law; it invites every man to become a law unto himself; it invites anarchy. To declare that in the administration of the criminal law the end justifies the means — to declare that the government may commit crimes in order to secure the conviction of a private criminal — would bring terrible retribution.” – Associate Justice Louis Brandeis