The CSI episode: How those testifying to the death of a person were able to identify the given body as dead. Even seeing that wild animals were eating the person is not enough to be sure he didn't make it. Likewise, more than 3 days after a death, depending on circumstances, risks decomposition impeding identification. Plus, the story of how one changed his appearance with wax - to the point of not being recognizable. Also, even though severe wounds will presumably lead to death, the person who is dying is not considered dead until he has truly died - to wit, he won't render anything impure until he dies (which doesn't mean he's going to live). The Gemara discusses how a seriously injured person who's been torn up might actually be saved. For example, cauterizing the wounds. Plus, what happens when one's injury is via a white-hot knife?