Yep. Thor was the King of Asgard. Thanos butchered Asgardian refugees, killed Heimdall with a spear, and murdered Loki by crushing the life out of him, all in front of Thor. From a purely view of a sovereign defending his kingdom, Thor literally executed a terrorist. However, modern views of law have reservations on targeted killing.

Targeted Killing Theory and Practice

There are two examples worthy of comparison to Thanos: Fleet Admiral Isoroku Yamamoto of the Imperial Japanese Navy and Osama bin Laden. Both were eliminated by the United States in targeted killings. Admiral Yamamoto was the architect of the attack on Pearl Harbor and the commander-in-chief of the Combined Imperial Fleet during World War II. The United States had Yamamoto killed in “Operation Vengeance,” which was carried out by two P-38s ordered to shoot down the transport carrying Yamamoto. See, Barber v. Widnall, 78 F.3d 1419, 1420 (9th Cir. 1996) and First Amendment Coal. v. United States DOJ, No. C 12-1013 CW, 2014 U.S. Dist. LEXIS 50663, at *22-23 (N.D. Cal. Apr. 11, 2014). Osama bin Laden was the terrorist leader behind the September 11 Attacks on New York and Washington, DC, plus numerous other terrorist attacks, who was killed by Navy SEALs in a raid. The justification for this killing is predicated on the fact the United States has a right to defend itself from terrorists.

The United States frowns on placing bounties for the assassination of leaders of foreign countries since the Civil War. PREEMPTION, ASSASSINATION, AND THE WAR ON TERRORISM , 27 Campbell L. Rev. 253, 259-260, citing Hugo Grotius, The Law of War and Peace (1625) reprinted in 1 Law of War: A Documentary History 16 (Leon Friedman ed., 1972). However, it is within the norms of war to kill a leader through ambush. Id. As the US Army in 1863 with the adoption of the Lieber Code in “Army General Orders Number 100”:

The law of war does not allow proclaiming either an individual belonging to the hostile army, or a citizen, or a subject of the hostile government, an outlaw, who may be slain without trial by any captor, any more than the modern law of peace allows such international outlawry; on the contrary, it abhors such outrage. The sternest retaliation should follow the murder committed in consequence of such proclamation, made by whatever authority. Civilized nations look with horror upon offers or rewards for the assassination of enemies as relapses into barbarism.

PREEMPTION, ASSASSINATION, AND THE WAR ON TERRORISM , at * 260.

Modern views on “Just War” require for a target killing that “the individual must be killed to save the lives of others, or to prevent great harm to others, and that capturing him or her will not have the same positive effect.” J Int Criminal Justice (2013) 11 (1): 47. This raises complex issues of proportionality, asking whether any due process concerns must be adjudicated if someone is targeted because of behavior alone, or behavior and status within an organization/nation state. Id. Conversely, the analysis is different for targeted killings of US citizens engaged in terrorist operations as explained in the DOJ whitepaper on targeted killings. The key question in both is whether the person targeted is a threat to the lives of others and that there is no non-lethal way to eliminate that threat. Id.

Does Thanos meet the Requirements for a Targeted Killing?

The Avengers confronted Thanos AFTER he had killed trillions of individuals with “The Snap.” Governments were barely functional across the universe. In comparison to the Nuremberg Trials after World War II, the Allies held those proceedings after they had secured victory and not defeat. Moreover, the governments of the Allied nations still functioned. The same cannot be said after The Snap.

Would killing Thanos have saved the lives of others? Likely yes, but not on the scale as before The Snap. Thanos had already completed his universal slaughter. Furthermore, he was gravely injured after using the Infinity Stones to destroy the Infinity Stones. That being said, Thanos had single handedly beaten up the Hulk, nearly killed Iron Man, killed Vision, and laid waste to Xandar in his quest for the Infinity Stones. Was he still a threat? Even with one arm, no one should count anyone that dangerous as NOT being a threat.

Justice requires a trial for anyone to be executed. Thor as the King of Asgard could have considered Thanos’s gloating as a confession, and Thor acted within his powers as a sovereign to seek retribution for his people. This would have been more in line with the targeted killing of leaders such as Yamamoto and bin Laden, but there are significant differences, namely neither had been captured at the time of death (however, Thanos had not surrendered). Thor’s actions overall do not comport with our understanding of targeted killings, but given the magnitude of death caused by Thanos across the universe, no one would complain about the lack of a trial. That does not make Thor right, but it does not make him wrong either.