That's not what I asked. I asked why is it morally wrong? Not whether it contributes to society. In my example, the person is a drain on resources; taking from the system without giving back. So why would it be immoral to kill him? It doesn't damage society at all; rather, it helps society. Similarly, is euthanasia wrong? In most cases, euthanasia would be applied to people that are, like the man in the aforementioned example, a drain on resources, either because of age or birth defects. Would it be morally wrong to kill them if it doesn't harm society?
Or: why is stealing wrong? After all, it's not actually harming the economy of a society as a whole; just... if you'll excuse the oft-uttered phrase, redistributing the wealth? Why does one man have any more right to riches than another man?
The speaker in that video would like to believe that morals arose from conventions designed to propagate civilization, but that belief cannot possibly apply to all morals, as I said above.