I'm sure the area where US laws apply can be discussed to greater lengths, but they're not applicable outside of the (geographical) US, regardless if you're a citizen or not.
Even if the subject of the action is an US citizen. (except for the taxation of non-resident citizens, which is a whole can of worms I'm sure)
This is for better or worse.
Extra-judicial killings happen a lot. In the case of self-defense for example. "Cop thought the suspect had a gun" is a mistake that happens often, but it can be legitimate as well.
I don't know why you think your constitutional rights disappear outside the geographic US. That is not the case. Constitutional rights protect citizens from our government regardless of their physical location. Indeed, geography was not a part of the Obama administration's justification.
As for your other point, self defense is a long established affirmative defense for murder, yes. However that has nothing to do with the issue at hand.
If the government has a case to kill a citizen, that case needs to be made in court and evaluated by a jury of peers.
Even if everyone has perfect intentions (doubtful), you need checks and balances to prevent human error.