Their not getting punished for killing hundreds of thousands in Yemen. Why should one more make a difference?
Yes, but those victims are Yemenis.
Which was the greater tragedy, fewer than three thousand killed in New York, Pennsylvania, and Washington, or several hundred thousand killed in Iraq? The answer is obvious, unless you take the absurd position that Iraqis are actually people.
This chap was a refugee, a resident of the US, who might have been applying for a green card, and with three children who were US citizens. So he's got to be worth at least as much as a few million Yemenis. Unless he did something the US doesn't like (looks like he won't have the opportunity now), in which case he might have been executed by the US instead.