At what point can a police officer stop using the excuse "I was just doing my job?"?

If a cop is asked to pull people over and take their money for not wearing the seat belt, most would say it is ok because the state legislature told him to do it. If a cop is asked to initiate the civil seizure of a vehicle because drugs were inside, even though the owner is later declared not guilty criminally and even though the drugs belonged to the penger, perhaps less would view "just doing their job" as an acceptable excuse. If a cop is asked by his superiors to kill innocent people because they are witnesses to police corruption, few would view it as an acceptable excuse. At what point does it become unacceptable. I ask it, not as a rhetorical question, but one where I would hope for an answer