Rent krasst skulle jag säga nej utifrån nedanstående text.
1. War crimes and crimes against humanity, wherever they are committed, shall be subject to investigation and the persons against whom there is evidence that they have committed such crimes shall be subject to tracing, arrest, trial and, if found guilty, to punishment.
2. Every State has the right to try its own nationals for war crimes against humanity.
5. Persons against whom there is evidence that they have committed war crimes and crimes against humanity shall be subject to trial and, if found guilty, to punishment, as a general rule in the countries in which they committed those crimes. In that connection, States shall co-operate on questions of extraditing such persons.
https://web.archive.org/web/20070211...law/guilty.htm
Osäker på om det finns någon mer tillämpbar eller mer "generell" lag för terrorbekämpning som USA skulle kunna rättfärdiga mordet med men lyfter denna så får någon rätta till eventuella felaktigheter. Nedanstående kommer från en amerikansk lag angående förhindrande och bestraffning angående terrorism.
Article 1
The contracting states undertake to cooperate among themselves by taking all the measures that they may consider effective, under their own laws, and especially those established in this convention, to prevent and punish acts of terrorism, especially kidnaping, murder, and other assaults against the life or physical integrity of those persons to whom the state has the duty according to international law to give special protection, as well as extortion in connection with those crimes.
Article 4
Any person deprived of his freedom through the application of this convention shall enjoy the legal guarantees of due process.
http://www.oas.org/juridico/english/Treaties/a-49.html
Min slutsats: Oavsett om lagarna är spot on eller ej, så bör det inte finnas tillräcklig legitim grund för avrättningen av Qasem Soleimani. En rättegång med personen i fråga tenderar att vara mest lämpligt eller att flera länder inom FN anser att det är berättigat.