Sanhedrin 78: Killing a Dead Man

With partial or shared causation, who is culpable? Which leads to a discussion of someone who is already mortally wounded or ill or injured, and then someone kills that person, when is that considered murder (and the perpetrator would be held accountable) - as compared to being exempt because there was no new "death" to perpetrate (because he was already dying)? And what if one's strike is not forceful enough, but someone else follows up? Or one's strike IS forceful enough, but happens not to kill? Plus, the position of Rabbi Nechemiah, and the assessment that the victim will live after all.