As an extrajudicial assassination that was not necessary to prevent an imminent attack, it was both illegal and unethical:
’In the wake of Suleimani’s assassination, Agnès Callamard, a Special Rapporteur on extrajudicial, summary, or arbitrary executions at the Office of the UN High Commissioner for Human Rights, noted that there is no oversight of targeted killings carried out beyond a country’s borders. The Executive simply decides, without any legal due process or approval by any other branch of government, who is to be killed. Accepting such an action makes it difficult to find any principled objection to similar killings planned or carried out by other countries. That includes the 2011 “Cafe Milano Plot,” supposedly masterminded by Suleimani himself, in which Iranian agents planned to kill the Saudi ambassador to the US while he lunched at a well-known restaurant in Washington, DC.
The only thing the US can say in defense of its assassinations is that it targets really bad guys, and the Saudi ambassador was not such a bad guy. That puts the rule of men above the rule of law.
The other justification that the Pentagon offered for the killing referred vaguely to “deterring future Iranian attack plans.” As Callamard pointed out, this is not the same as the “imminent” attack required to justify acting in self-defense under international law. She also noted that others were killed in the attack – reportedly, a total of seven people died – and suggested that these other deaths were clearly illegal killings.
A careful reading of the transcript of the January 3 press briefing, held by three unidentified senior State Department officials, reveals the Trump administration’s real thinking. In response to repeated questions about the justification of the assassination, one official compared it to the 1943 downing of a plane carrying Japanese Admiral Isoroku Yamamoto, who was visiting Japanese troops in the Pacific – an incident that occurred in the midst of war, more than a year after the Japanese attacked Pearl Harbor. Another official said: “When I hear these questions it’s like you’re describing Belgium for the last 40 years. It’s the Iranian regime. We’ve got 40 years of acts of war that this regime has committed against countries in five continents.” At one point, the official who had compared the assassination to the killing of Yamamoto blurted out: “Jesus, do we have to explain why we do these things?”2
If senior State Department officials believe that the US is engaged in a just war with Iran, as it was with Japan in 1943, the killing of Suleimani makes sense. According to standard just war theory, you may kill your enemies whenever you have the chance to do so, as long as the importance of the target outweighs the so-called collateral damage of harm to innocents.
But the US is not at war with Iran. The US Constitution gives Congress the sole authority to declare war, and it has never declared war on Iran. Speaker of the House Nancy Pelosi suggested that congressional leaders should have been consulted on the plan to kill Suleimani. If it was an act of war, she is right.
If, on the other hand, the killing was not an act of war, then, as an extrajudicial assassination that was not necessary to prevent an imminent attack, it was both illegal and unethical. It risks severe negative consequences, not only in terms of escalating tit-for-tat retaliation in the Middle East, but also by contributing to a further decline in the international rule of law.…’