Why is it that there are cops whom although as a cop, they did their job well done, as in they arrest...?

... or place a charge at any person who does a crime (be it minor or major offence) & don't give a damn care or mercy to them if they ask for a second chance, but on the other hand, they (the cops) themselves broke the law, as in for example, they help (or should I say conspire) their close friends who are underage to buy for them cigarettes, alcoholic drinks &/or lottery tickets??