Fallacy:
"We should act so that utility is maximized."
Response:
There are many problems with utilitarianism. First, measuring and comparing utilities is not actually possible (Mises)—it's all hypothetical. This makes it impossible to impose in practice; but as with most state justifications, it is used as excuse based on the ruler's decision of what people's utilities are. It is an amoral (hence frequently condoning immoral infringement of rights) philosophy than can justify even so far as robbing and murdering some so that utility across a population is increased; it does not respect NaturalLaw.
(Refer also Rothbard's arguments in chapter 26 of The Ethics of Liberty.)
Not only are utilities not comparable nor measurable, they are also subjective. One person may calculate (or, rather, assert the belief) that the sum of utilities in killing all ethnic X (loss from all X, gain from all non-X) can be used to justify genocide. Another may deny it. A philosophy that asserts both A and not-A denies the law of non-contradiction (both A and not-A may not be true at the same time). And since utilities are not actually calculable, the calculator—the "measurer-and-decider-in-chief"—is not even actually determining utilities, but using utilitarianism as a smokescreen for their preferences. Having such a "measurer-and-decider-in-chief", of course, begs the question (of right actions) by smuggling in this decider without justification.
While past abuses do not invalidate utilitarianism, they do put another nail in a coffin that is already well-studded and deeply buried: tyrants of every stripe have used utilitarianism to justify their acts—a Master Race, Lebensraum, purification, and so forth.
Some may propose a variation on utilitarianism (for example, "no killing") to attempt to get around the horrors the philosophy justifies. However, this weakening is much like attempts to circumvent Gödel's incompleteness theorems by weakening the formal system in question: either you weaken it to be trivial (parallel: not powerful enough to justify the powers you assert the state must have), or it is still inconsistent (parallel: justifies genocide).
There is also, of course, the problem of justifying applying utilitarianism in the first place, i.e., why is it alright to harm some if others benefit?
Also, a funny related comic.