Now you've misplaced the point. No one said man was special "just for existing," man is special for his potential, and that potential is worth something, and it is a crime against human nature not to tell children about it, but instead raising them to wallow in their own hopelessness and cynicism about their species' incidents of succumbing to unreason.
What difference does any morality make, if there be any one or group whose morality differs? Some moralities love their neighbours, others eat them. What's the difference other than might making right?--or the admission that there is no morality after all and we might as well be like whatever animal we choose in our dealings with, or exploitations of, others?Because humans think it is wrong for a man to kill his neighbor. For certain values of neighbor. This is a very straightforwards point; human morality is a thing made by humans to govern human actions. Ergo it is immoral if it goes against human morality.
(Well, human moralities; there's never been just one, and pretending otherwise is very dangerous.)