I now had time to look into the matter. It seems people here who assert it to be legal are quite off:

https://lieber.westpoint.edu/assassination-law-of-war/

Although the term treachery did not feature in the article, the sense that certain acts violate the law of war principle of chivalry, which is reflected in the work of the earlier scholars, was clear. In Article 101 of the Code, however, the term did appear, demonstrating its centrality to how war was not to be fought: “While deception in war is admitted as a just and necessary means of hostility, and is consistent with honorable warfare, the common law of war allows even capital punishment for clandestine or treacherous attempts to injure an enemy, because they are so dangerous, and it is difficult to guard against them.” In subsequent national and international codification efforts, treachery assumed a place of prominence in the treatment of assassination, as it had historically.

The most recent comprehensive treaty governing the conduct of hostilities is the 1977 Protocol Additional (I) to the Geneva Conventions of 12 August 1949, and relating to the Protection of Victims of International Armed Conflicts. Additional Protocol I’s Article 37 styles acts during an armed conflict that were previously labeled assassination as “perfidy.” The article confirms again that the essence of the prohibition is treachery, not mere deception or trickery.

1. It is prohibited to kill, injure or capture an adversary by resort to perfidy. Acts inviting the confidence of an adversary to lead him to believe that he is entitled to, or is obliged to accord, protection under the rules of international law applicable in armed conflict, with intent to betray that confidence, shall constitute perfidy. The following acts are examples of perfidy:

(a) the feigning of an intent to negotiate under a flag of truce or of a surrender;

(b) the feigning of an incapacitation by wounds or sickness;

© the feigning of civilian, non-combatant status; and

(d) the feigning of protected status by the use of signs, emblems or uniforms of the United Nations or of neutral or other States not Parties to the conflict.

Note that the examples refer to the second sentence, to give someone false security by suggesting he would be entitled to protection under laws.

Blowing someone up with an explosive disguised as a civillian item in a civllian environment seems to be quite perfidious.

Assassination during wartime denotes (1) the treacherous, (2) wounding or killing, of (3) individual adversaries, in other words, perfidious attacks. Although the two terms often appear in the disjunctive, it is also reasonable to include outlawry, such as putting a price on the enemy’s head, within the scope of the definition assassination, as was done in the Lieber Code and suggested by Greenspan.

Military manuals have occasionally suggested that assassination is limited to non-combatants or requires a particular mens rea. […] However, the historical intent of the prohibition during armed conflict was to encompass the treacherous killing of the enemy, not just non-combatants. This is clear from the Hague Regulations’ Article 23(b)’s reference to “individuals belonging to the hostile nation or army,” which would include civilians and members of the armed forces. Additionally, the prohibition of assassination contains no mens rea requirement beyond an intent to betray a confidence or to encourage others to kill the individual(s) by placing a price on their head.

It is perfectly reasonable to speak of a murder here.