Um, unless you are at war with any of those countries, you go and kill someone, you are a murderer, plain and simple.
I think the point is that in some countries, the police are not protectors but instead they are oppressors. For instance, in Nazi Germany the police would be involved in rounding up Jews so they could be shipped to concentration camps - killing them would be fighting for liberty.
I'm not sure about all of the countries, but definitely not in the USA.
Depends what they are doing. If they are just directing traiffic, you go up and shoot them, you are a murderer. If they are physically threatening you like the Nazi German police you described then it is justified to kill in self-defense of course.
