It's not possible to know something that only exists in the realm of the dead. How do you know that, nobody knows that. In the future that could change. Using science and technology.
That's why I put know in quotes. You have to be a physical being to know something (same point), you have to have a functioning mind (and again) When you die we don't know what happens other then your body stops working, what that person sees or experiences is anyone's best guess. Unless we can kill someone and then bring them back to life there is no way to prove it. You don't know what we can do in the future, we could create a whole universe with life and death within it and observe from outside and within it's function and know everything about it, in fact it might be a toy for children, how can you so categorically make these statements?
Science needs a testable hypothesis in order to work up a sound theory, since the hypothesis is untestable without killing then reviving someone somehow then we will never know. And even then, would that even be ethical? Probably not.
Again it might not have to involve killing or reviving, even if it does the universe might not care what is ethical, only some people at this point in time, and the right or wrong of it can be argued either way.