Wondering if there was any canonical precedent that could provide an answer:
In “Birth of a Dark Hunter”, after the Dark Hunters surrender and Nidhiki’s betrayal is revealed, Lhikan tells Nidhiki “Get out before I kill you.”
If Lhikan had killed Nidhiki, would that have been a violation of the Toa Code? The Code permits Toa to kill during wars, but at that point the Dark Hunters had surrendered. Would killing Nidhiki be considered a war crime? (and is there a concept of war crimes in Bionicle?) Would such an act by Lhikan be worthy of banishment to the Pit?
In any case, in the short story, The Shadowed One makes it sound like Lhikan didn’t spare Nidhiki out of regard for himself (e.g. to avoid violating the Toa Code or ending up spirited away like Tuyet), it was because Lhikan thought by letting him go he was doing Nidhiki a favor.