In this context I think the word "asesinado" is a better option, but that's only my opinion.
Actually in Spanish we have the "dar muerte" verb and its past participle "fue muerto", that would be the closest translation for "murder" and "was murdered".
Why I think "asesinado" is a better option? because of the type of murder.
Generally a murder/dar muerte is an act of killing someone intentionally, but generally is in a confrontation.
In Spanish there's a huge difference between "matar" and "asesinar": normally "matar" is for non human beings and when is not the intention
while "asesinar" is the act of killing someone with intention and even advantage.
So under this context for me killing by betrayal sounds much more like an assassination.
But "dar muerte" would be also correct in the general context and for the same reason that this is a legend and the circumstances are disperse "fue muerto" could be the right option.
Let's go with the case of Jesus Christ. We could argue that he was killed, murdered or assassinated and the three options would be correct because the conditions of his death (pretty much the same as Osiris).
In the end the usage will fall depending of each person point of view:
1.- Accidental/Required : Matar
2.- Confrontation and with intention: Dar muerte
3.- With Intention and Advantage : Asesinar
For a lot of people a bullfight is an act of assassination, for others is a confrontation with the intention of killing and the survivor is the winner so it could be : "fue muerto" or "asesinado"
The interesting case of a man fight:
In a box fight as it would be accidental it would be : "fue matado", but due matado has the animal connotation and would show a lack in respect it is better to just use "murió", but friends, family and even the opponent would feel like "lo maté/mató", but never feel like "asesiné/asesinó"