If you painlessly kill everyone in the world but that one happiest guy, the _average_ happiness increases.
If you say that the only the "total" happiness matters then would you rather have a world with 6 billion marginally happy people or a world with 200 million very happy people, if the total happiness of the 200 million is somewhat less than the total happiness of the 6 billion, even though each individual of the 200 million is much happier? Personally I would rather, all other things being equal, have the 200 million people world. And what about unhappy people--if someone is unhappy that presumably would detract from the total happiness, so it would be reasonable then by the utilitarian ethic to murder the depressed if there were no consequences but their death, even if those depressed do not actually want to die.
Happiness is certainly one goal, but I believe a bigger goal than happiness is fulfilling the expression of each individual's potential. If an alien were to offer me bliss in exchange for most of my mind, I would not take it. Would you?