HENRY HILL IN GOODFELLAS (1990)
Mob boss Paulie Cicero (Paul Sorvino) and longtime partner Jimmy Conway (Robert De Niro). After Cicero tells him to stop selling cocaine, Hill (Ray Liotta) continues to, and eventually the Feds nab him. When he realizes that Conway intends to whack him so he can't turn snitch, Hill takes it as a convenient excuse to flip on everyone he knows and save his own sorry ass.
Punishment for snitching:
Has to spend the rest of his life in the Witness Protection Program as an "average nobody" eating Middle American pasta sauce that resembles ketchup.