This is from the ICRC (Red Cross) website:
‘Derived from Common article 3 to the Geneva Conventions, the notion of “direct” or “active” participation in hostilities is found in multiple provisions of international humanitarian law (IHL). Direct participation in hostilities by civilians entails loss of immunity from attack during the time of such participation and may also subject them, upon capture, to penal prosecution under the domestic law of the detaining state. Despite the serious legal consequences involved, neither the Geneva Conventions nor their Additional Protocols include a definition of what constitutes “direct participation in hostilities”.
Even though it is not a violation of IHL for a civilian to fight for his or her country, the lack of combatant or prisoner of war status implies that civilians directly participating in hostilities may be prosecuted under domestic law for their acts regardless of whether or not they violated IHL. It is not clear, however, whether domestic criminal prosecution could ensue for the mere fact of directly participating in hostilities or whether such participation must involve an act prohibited under domestic law or international law.’
Despite the ambiguities mentioned above, it seems pretty clear that in the scenario of this question, the killer would be subject to prosecution for murder under domestic or international law.