That doesn't line up with the Constitution, which clearly states what the US Government can do (I know, people commonly mistake the Constitution for a list of rights for the people rather than a list of exceptions for the government as to how they may or may not act).

The Constitution grants the right to due process for citizens. It doesn't place a limit on where that citizen is currently resided.

Killing someone in combat is one thing. The same as if you are forced into a firefight with someone as a police officer. Targeting and assassinating someone who is not immediately and directly threatening realistic harm is another - and that is what has happened, so far.

Stating "well, if they're outside the geographical boundaries of the country (or, not at least 100 miles in from the shores and borders of the US, apparently) then they're fair game" is absurd.