I think what distinguishes this from other murders is the shared culpability in the wrongful death.
Normally, the guilt is 100% with the murderer. Off the top of my head, I can't think of any circumstances in which someone might contribute to his own wrongful death. People do contribute to their own justifiable deaths all the time, when they are killed by someone protecting his own life or that of someone else, or killed in the process of committing some other crime. In such cases, the killer isn't prosecuted, because the killing was justifiable - the blame for the death lay entirely with the deceased.
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