Andrew
The problem with that is what you now see playing out in US political and the question around presidential immunity.
Imagine the family of the killed guy going after the president for murder. Of course they have no standing in court because they don't have political influence. But legally this is very similar to the question 'can the president order seal team 6 to kill a political rival and claim immunity?'. It is about the president making a decision that bypasses law and justice.
What if Biden has Trump assassinated,? Clearly Trump is a huge danger to the entire fabric of the democratic institutions and rule of law and - given his statement to not support NATO countries under attack - to the entire world order and playing into the hands of Putin, Xi and Kim Yong Un. He is a much larger threat than al-Awlaki could ever be. So if one is ok, why not the other?
If you open pandora's box by allowing it in one case, it becomes incredibly difficult if not impossible to define limits of such killings.