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.