Snitched on: Corrupt police officers. Sick of watching his fellow cops steal, take bribes, and physically abuse suspects, principled police Serpico (Al Pacino) goes to higher-ups and, when they refuse to do anything, the press.
Punishment for snitching: His backup officers provide little help when Serpico takes a (possibly pre-arranged) bullet to the face on a narcotics sting. After surviving and testifying in front of the Knapp Commission on corruption, he flees to the safety of Switzerland. You see, the police take this no snitching shit as serious as crooks. Fucking hypocrites.