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.
By your logic, it's acceptable for the PM of Italy to order the assassination of an Italian-American dual citizen because that Italian has pledged allegiance to a foreign state that engages in violence?
If that dual citizen is actively engaging in such violence and Italy is a (potential) target of such violence, and his activities are happening outside Italy, yes.
> "Where high-level government officials have determined that a capture operation is infeasible and that the targeted person is part of a dangerous enemy force and is engaged in activities that pose a continued and imminent threat to US persons or interests."
Responding to the above, Obama did do something: he presided over the first extrajudicial assassination of a US citizen by drone strike.