I would draw the comparison to someone holding a hostage at gunpoint. You don't put that person on trial before final judgment comes down.
I would draw the comparison of someone holding a hostage at gunpoint, so cops go and kill an innocent guy in neighborhood that is pretty much unrelated with the person holding the first as hostage. I'd definitely want to setup a way to judge those incompetent cops.
...better have a judgment from a jury, proved beyond a reasonable doubt, after considering the necessary evidence that satisfied any warrant requirements and related rules.
Failure to satisfy this due process - even when the target is a well-known criminal who has fled to another country - could be evidence of the murderer having violated their oath to office. It's certainly not a quality we should allow in our politicians, officers, or civil servants.
/yes, I realize that's not the antecedent for "someone" you intended
Bob is actively trying to kill Americans and is avoiding trial by hiding in a foreign country. Joe is a peaceful guy who's falsely accused of trying to kill Americans, and lives in a foreign country for unrelated reasons. By what system do you propose we distinguish between Bob and Joe?
Now how should we make sure that they are indeed trying to kill American citizens before we punish them?
It should be fairly simple to find a single charge made by the US government that he was killing american citizens...