The arguments given by people to justify unethical acts are usually utilitarian, which is to say that a given act is defended on the grounds that it is beneficial to someone. This is commonly manifested in the arguments given in defence of the alleged "need" for a whole host of economic policies, and government in general. For example, conservatives often argue that the tax-funded defence industry helps "stimulate the economy". It is often argued that the public sector is justified because it "creates jobs". So long as a measure can be shown to be beneficial to a specific group of people, the utilitarian is prone to be comfortable with it.
On one hand, these kind of arguments are fallacious even in utilitarian terms in that they ignore the cost side of the equation. Government creates only government jobs, which inherently comes at the cost of private jobs. Governments cannot increase employment in one sector without withdrawing it from another. All government jobs represents a net loss to the tax-payer. Government cannot create wealth, it can only redistribute it, and in the process of redistribution it actually decreases overall utility by shifting production into consumption. The government itself does not produce, it consumes from private production. It is a leech on the producing classes, which includes both workers and genuine entrepreneurs.
On the other hand, even if it can be substantiated that a given measure is beneficial to people, this does not necessarily justify it in ethical terms. After all, one can try to argue that a thief is justified in their theft because they donated the stolen goods to charity, but that would not justify theft. Just because something may be beneficial to some people does not necessarily mean that it is justified, nor does it negate the fact that it may very well be at the expense of other people. Economic efficiency and metric benefit is not a proper measuring stick of justice. In short, the ends do not justify the means. One can very well show how a redistribution benefits certain people, but that would not justify confiscation of property.
A common mistake made by many utilitarians is the broken window fallacy. The broken window fallacy refers to a situation where one argues that a destructive act is justified because it may lead to the gain of others in some way, usually by stimulating economic activity. Should we encourage children to break windows of baker's stores because this stimulates the economy by making the baker buy a new window? Or are destructive acts never justified, and this actually represents a loss to the baker? To take the former view ignores how those same resources would have been used otherwise. This fallacy is precisely what is going on when people claim that warfare benefits the economy, ignoring that there are immense costs that can only be delayed at best.
While some utilitarians think more long-term than others, utilitarians may often take a short-term view, being concentrated on obtaining the maximum utility in the present, regardless of long-term consequences. A good example of this is manifested in monetary policy. The establishment view goes roughly as follows: "monetary inflation is a good thing because it stimulates the economy by raising wage rates, creating jobs and stimulating growth". But as Ludwig Von Mises demonstrated close to a century ago, in the long-term this is unsustainable, it must be reconciled in a downturn, malinvestments must be cleared and the debt it generates must be payed off. This is beside the fact that the inflation also reduces the value of each monetary unit, thereby diminishing the purchasing power of the wages, which manifests itself in a higher cost of living. Viewed in ethical terms, all of this is irrelevant to the question as to whether or not the stealing of the value of people's money can be justified.
Also, utilitarianism is all about maximising "happiness". But the problem is that there is no concrete definition of happiness, which is to say that it varies from individual to individual. The question becomes "who's happiness"? It is not really possible to statistically measure "happiness" in the first place. In practise, therefore, it seems as if the utilitarian is stuck either arbitrarily trying to measure things in terms of their own definition of happiness (which makes way for authoritarianism), or playing the role of a value-free observer that accepts whatever a person's own definition of happiness is (which makes way for hedonism). Another route that may be taken is to define happiness by whatever a majority defines it as (which makes way for persecution of the minority).
Furthermore, and this is very important to stress, happiness is not the criteria by which we measure right and wrong. The happiness of a murderer may come from murdering, but surely we do not condone murder simply because it brings happiness to the murderer. Pleasure may seem like a good goal to strive for, but some people may find pleasure at the expense of others. In some cases, commonly accepted morality may very well require that people abstain from acting out of primitive desire for pleasure. A man may find pleasure from sexual intercourse, but in order to be ethical they must abstain from simply forcibly mounting every woman they see. And if utility is defined more in terms of general economic well-being, a person may steal in order to stay alive, but in order to remain ethical even the most impoverished person must abstain from robbing banks.
In short, it is impossible to consistently apply any ethical principle using utilitarianism as a method of looking at things. Any ethical consideration can in theory be overturned using utilitarianism so long as it is perceived or can be sufficiently proven to be net beneficial or bring happiness. At best, utilitarianism can be used to show how certain actions will have negative or unintended consequences. It can have a limited use in this respect. But as an ethical system it is a nightmare. Pragmatism becomes more important than principle and even if a long-term view is held a utilitarian may find ways to attempt to justify just about anything. In particular, so long as something can be shown to benefit a larger amount of people, the individual or minority is fair game to be trampled upon in a utilitarian world. This is fundamentally detrimental to the cause of individualism.
A fundamental requirement for justice is universality, which is to say a logically consistent application of principles to each individual. Utilitarianism cannot possibly consistently apply a principle to each individual, for it seeks a numerical maximisation of variables in which whoever has the most numbers wins. It turns everything into a numbers game, into a game theory of a sort. There is a fundamental clash between universal justice and bare consequentialism. Universal justice proclaims that certain things are wrong regardless of the consequences and regardless of the amount of people involved in or benefited by them; even if it's one individual against the world. Utilitarianism, on the other hand, essentially proclaims that anything is right provided that it has good consequences; even if the individual or minority must be forced to sacrifice for a "greater good" (whatever that means).
One's options are as clear as a bell: either ethics are definitive and universal, or they are prone to juxtaposition and subordination based on consequences and sheer pleasure.