So, there is a man, Gibril Massaquoi, on trial in Finland for things he allegedly did during the Liberian Civil War. Among the charges is murder.
Here's the thing: in war, it is generally to be hoped that everyone involved has premeditated ending the lives of their opposite number. This would make every soldier ever to serve in war a murderer according to civilian rules.
So, what is the cutoff for soldiers in an active war to commit murder?
As with all international law, it depends on "who says so": I will draw on the Geneva Convention Relative to the Protection of Civilian Persons in Time of War as most relevant. Article 3 distinguishes combatants from non-combatants, saying that
Persons taking no active part in the hostilities, including members of armed forces who have laid down their arms and those placed hors de combat by sickness, wounds, detention, or any other cause, shall in all circumstances be treated humanely, without any adverse distinction founded on race, colour, religion or faith, sex, birth or wealth, or any other similar criteria.
They then specifically prohibit murder:
Violence to life and person, in particular murder of all kinds, mutilation, cruel treatment and torture
but this only applies to non-combatants. There simply is no prohibition against killing combatants (unsurprisingly). Liberia acceded to that treaty, so for instance if one of their soldiers murdered a non-combatant, in principle they should try that soldier for the crime of murder under Liberian law. If some random dude (not a soldier) murders anybody in Liberia, in principle they should try him for the crime. Soldier may kill enemy soldier, even when the killed soldier is sleeping and poses no immediate threat to the soldier who kills him. That's the nature of war.
In the case of Massaquoi, he might have been prosecuted by Sierra Leone, but negotiated immunity in Sierra Leone in exchange for information on his RUF colleagues. There was no such tribunal or arrangement w.r.t. his involvement in Liberia, and Finland opted to conduct an extraterritorial trial based on war crimes (not the killing of combatants). His acquittal was based on the lack of evidence that it was him that did the reported deeds (I don't know if there is a publicly available judgment, but it is 850 pages and in Finnish, so toivotan onnea projektille.
