What oath did you mean, where they are swearing to kill the people they're ordered to? I ask because even in the military, modern democracies actually make a big deal out of loyalty to the law instead of purely to your commanding officer -- like if your commander orders you to shoot civilians for no reason, and you do it, you can both wind up in a court-martial together. There was kind of a big deal one specific time about whether or not acting under orders was a defense after the fact and what the person was supposed to do in that situation. I have studied history US and otherwise, yes; I suspect a lot more than you think I have.
So where in the police oath does it say about killing the people you're ordered to kill? You're lecturing me to be honest, to some extent, on how it is, so I'm asking you, hey can you tell me some more about this thing you told me is how it is?