Assuming the best possible thing is happiness, (because, after all, everything a person does is to acquire that), if everyone was connected to an IV which injected a chemical that causes complete happiness, wouldn't that be the best possible life? And wouldn't killing them not be a crime, since the only reason murder is "wrong" is because we instinctively fear death, and these people would not have instincts, and would therefore be the equivalent of robots? Since they wouldn't know that they're about to die, they'd be happy until they'd cease to exist - and once they cease to exist, they can't be unhappy. For that matter, no one would volunteer for such a type of happiness, since such happiness would be equivalent to ceasing to exist. So why are happiness and life inherently good? Are they inherently good? Why is it bad to murder someone? Are morals at all important? And so on. In other words, happiness does not semantically equal good. Happiness is a completely different concept, which cannot be considered either good or bad. And therefore, preventing someone's happiness is not bad. Disclaimer: For no good reason whatsoever, I also enjoy happiness. (Sorry for the semantics there.) And I'm not depressed. It's just that I'm a thirteen-year-old.

You ask a whole bundle of questions at once -- more than can be tackled in my response. Let's focus on two interrelated strands of your question: the badness of death and the wrongness of killing.

Suppose we could use chemical measures to make people (as you say) completely happy. You say such people would not have "instincts" and would not fear death. Both claims strike me as very implausible. A happy life seems rather to require instincts: instincts to pursue and engage in various activities that make us happy. Furthermore, I see no basis for thinking that a 'completely happy' person would not fear death. Exactly why people fear death is hard to pin down. But one plausible explanation is that they fear because death entails losing out on the further goods of life. But a completely happy person would have more reason to fear death, since (after all) they would stand to lose more than less happy, or unhappy, people would.

So I find the initial thought experiment to be, well, perhaps not impossible -- but at the very least, implausible. Setting that aside, you then hypothesize that it would not be morally objectionable to kill those who have become completely happy due to these chemical measures, on the grounds that (a) they would not fear death, and (b) the fear of death is what makes killing wrong. As the previous paragraph indicates, I don't think (a) is very likely. But even if (a) were true, (b) is unlikely also. A simple thought experiment: Your neighbor has achieved a kind of equanimity about death, reconciling herself to the fact that it is inevitable and nothing to fear. Does it follow that there are no moral reasons against killing your neighbor? Unequivocally, no. Again, your neighbor may stand to lose a great deal from her premature death (and of course there are those others who would also be detrimentally affected by her death).

In short, I don't see that your thought experiment is represents a very coherent possibility, and even if it did, it would not seem to have the implications you entertain.

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