Look, as discuss this, it might be useful to read up on the extant case. It's not like al-Aulaqi was surprised that the US was trying to kill him. And it's not like the question wasn't ever considered by the courts. And it's not like the Obama administration was particularly secretive about it. Hell, the UN listed him as a terrorist. Canada froze his assets. In 2010, al-Aulaqi's father filed a lawsuit in 2010 trying to get an injunction against the administration.

Etc. & so forth.

He could have turned himself in.