There is a significant moral question related to the Trump Administration’s characterization of this killing as a
deterrent.
The statement released by the US Department of Defense…
… contains this statement:
This strike was aimed at deterring future Iranian attack plans.
Soleimani, the one who was killed, is not the one being deterred. It is other people, still living, who may (or may not) be deterred.
The justification of killing in a just war normally assumes that the person killed was the same person who posed a threat to human life. In that case, the killing directly removes the threat.
Killing for deterrence removes this link. The killing does not directly remove the threat, but merely intends to influence others who pose their own direct threats.
The justification of killing in a just war also requires that there be a reasonable expectation that the threat to human life will be significantly reduced, and that no other means are reasonably available to remove that threat. In the case of the killing of Soleimani, neither of these is certain; it is not a sure thing that others will be deterred, and it is not clear that other (more direct) means were not available.