It's not a matter of wanting to. No one does, but having to. Do you think this theoretical discussion is any different than the actuality that happened to the Jews in Nazi Germanny? The russian citizens who were takes to die in the gulags? The Armenians who were sent on forced death marches in Turkey? The intellectuals taken to their deaths in Cambodia? I wonder if you could ask them the same question what their answers would be.
And BTW the Nuremburg trials established that following orders is no excuse even for military. It is far less of an excuse for police. They can just quit with no consequences if they don't like their orders. They wont be shot for desertion. I'm guessing that most of them won't have any problem with following those orders, and therefore there should be no moral dilemma about shooting them.
Different times, different circumstances, different places, and mostly, different people.
Good point about the Jews and others who gave up their firearms under Hitler. People who give up their firearms are dead people.