By your implication, an extrajudicial execution by the executive rather than the legislative branch does not constitute attainder. It does, and is prohibited.
That is correct.
It does, and is prohibited.
No, it does not. Bills of Attainder are, quite specifically, acts of the legislature.
This actually does not matter in this case: the 5th Amendment prohibits the executive from simply authorizing killings (prohibitions against the taking of life or liberty w/o due process) and the 6th explicitly states that all criminal cases have the right to a jury trial (and to mount a defense). If these killings are not the result of criminal law, then they must be civil law, and the 7th Amendment puts limitations thereon, specifically: jury-trial in any case wherein the amount in controversy exceeds $20. (And of course your life is worth more than $20.)
So, in any case these 'authorizations' of killing are utterly repugnant to the Constitution AND Murder.