When did you prove that pareto optimal is a necessary condition of the best economic outcome? I offered a counterexample: taking the ill-gotten gains from the rich to give to the poor would be best, on moral grounds, if the economic regulations have been tainted by virtual plutocracy, allowing the rich to get away with murder. That "sacrifice" of the rich minority would harm the rich but only by pursuing higher justice. So that would be a case in which utilitarianism would trump pareto optimality in our pursuit of the best economy. It's just that scientistic economics has trouble incorporating these higher values of fairness and justice.

There's an ambiguity in "making someone worse off." Is that supposed to mean taking some of that person's property illegally or immorally? A situation is pareto optimal if the only way to improve some people's condition is by harming someone else. It's just the denial of the utilitarian's point that some sacrifices are justified, as Chang said.

But is the rich person harmed in the Robin Hood scenario (in which you "steal" from the rich to give to the poor, to compensate for the much worse stealing that the rich perpetrate, which latter stealing happens to be legal because the whole society is unjust on moral grounds)? Technically, yes, if you tax the rich person to redistribute that money to the poor, you're making that rich person worse off, at least in some narrow terms. But you could be acting justly, according to a moral sense of what's right, counteracting the plutocratic tendencies of the warped society. So that "harm" would be in the service of a greater good, which is precisely the utilitarian's point.

Also, if the rich person has more money than God, are you really harming him by taking what he'll never miss?

Again, as Chang said, this is a political, philosophical disagreement about values. So by saying pareto optimality is a necessary condition of the best economic outcome, you're presupposing a philosophy that disagrees with the utilitarian, Robin Hood counterexample I've laid out.