US Person is not arguing whether or not Soleimani qualified as a "terrorist" as currently defined by US law. For the sake of this argument that category is granted. The killing took place in a third country, not inside Iran. If it did it could be argued that Soleimani's elimination was an act of war. Significantly the current defense secretary, Mark Esper testified at his confirmation hearings that neither the AUMF of 2001 (against those responsible for 9/11 attacks) nor the 2002 AUMF (against Saddam Hussein's regime) would legally allow military action against Iran. Thus, an attack against Soleimani inside his home country would simply have been an unauthorized act of war. Not since the Korean Conflict has Congress cared much about exercising its constitutional prerogative to declare war. And even if Soleimani was considered a terrorist, he had nothing to do with the 9/11 attacks or Iraq's previous regime--in fact he was its enemy.
Killing Soleimani inside Iraq is a whole different case, because doing so is arguably a murder under current US criminal law. US Person is not alone in this conclusion. An ex-CIA lawyer, Vikki Divoli, writing at the Daily Beast thinks so, too. Let's look at Section 1116, Title 18 of the US Code. it says:
Whoever kills or attempts to kill a foreign official, official guest, or internationally protected person shall be punished as provided...[herein] “Foreign official” means—A Chief of State or the political equivalent, President, Vice President, Prime Minister, Ambassador, Foreign Minister, or other officer of Cabinet rank or above of a foreign government or the chief executive officer of an international organization, or any person who has previously served in such capacity, and any member of his family,....