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.