I am an anarchist for ethical reasons, but I also believe that it superior from a practical perspective, though utilitarianism is not a valid justification for anarchism, simply an explanation of it.
Here, ethics is a system of rights that human possess as derived from two principles. One is the principle of self-ownership. You own your body and mind and can own property external to yourself. The second is the non-aggression principle, that it is ethically illegitimate to use force against others, violating their rights. From these two principles, we deduce all rights as property rights. Contracts are voluntary agreements handling the rights of multiple people. Within this framework, only individual actions can be justified as ethical or unethical. An ethical action is one that does not violate the rights of another, either by having nothing to do with another person, or if the action involves multiple people, by being voluntary. An unethical action is one that is not voluntary and violates a person's rights. Examples of unethical actions are murder, theft, assault, rape, fraud, breaking a valid contract, etc.
The state is, by definition, incompatible with rights. This is because the state is a monopoly on force in a certain area. It is not a voluntary institution. You must do as the state commands, regardless of ethicality, or force will be used against you. Your property can be stolen, you can be put in a prison, or even executed even though you violated nobody's rights. This makes government inherently unethical.
An anarchist of this type, usually referred to as a voluntarist or anarcho-capitalist, is not simply in opposition to government, but in opposition to all force, and the state is simply the most powerful, organized form of force.
This is not to say I am a pacifist or that I do not believe in justice. If a person threatens you or attacks you, you have the right to defend yourself, with lethal force if necessary. If someone violates your rights in some way, you have the right to seek restitution.
I believe that government is evil because it is the worst offender against rights. The common justification for government is that it is put in place to protect the rights of its subjects. This is a flawed justification because the state does so by using force. It must violate the rights of its subjects. We are attempting to protect rights by violating them, so the state is in no way ethically justified.
This is the ethical justification for anarchism. One concerned with these or other problems may see the state as necessary to have a functional society and advocate a minimal government. These people, in contrast to ANarchists, are MINarchists. Minarchists often accept that for practical reasons, private, voluntary organizations are always preferred to government organizations. This is central to the economic arguments made by libertarians advocating for a smaller government. They accept that the market functions better than the government. The anarchist simply takes this a step further and applies the same principle to all government functions, including police, courts, security, law.
The specifics on how such a society might function are not necessary to the argument, though various proposals are often made. The reality is that we cannot know exactly how such a society would function or how many different ways it can function. This, like everything else, is left to the voluntary market to decide.