This reminds me of a famous quote coined by the famous comic book artiste Alan Moore which is, "quis custodiet quie custodiet" which is Spanish for "Who Watches the Watchmen" (refers to his famous graphic novel and cinema film as well)

Also I believe it was Abbie Hoffman (famous 1960s peace activist) who famously is quoted as saying, "let he who fights dragons be careful not to become a dragon himself, and if he shall stare into the abyss, so too may he become an abyss" - so that basically means if you fight criminals all day even with the best of intentions, the wear & tear of it over time might eventually turn you into a criminal too