I don't know what incident you're talking about.
The only killing of a US national by the Obama administration I can recall, is when Anwar al-Awlaki was killed using a targeted drone strike in 2011.
But in al-Awlaki's case he was 40 years old, had left to join al-Qaeda, was their main English language recruiter,.and had been involved in planning several attacks.
As Obama put it, his citizenship was no more of a shield than a rooftop sniper being taken out by a member of the SWAT team.