When the state locks people up it is denying them their freedom. When the state executes someone it's denying them their right to life. If a private person did either of those things they would be committing a criminal offence, why isn't the state liable when they do? If the state can be found guilty of breaking a contract (which it can) why cannot it be held liable for the punishments it inflicts on others?

There are answers, of course, there are answers. John Locke, as a prime example, produced the seminal work on the rights and duties of the state and its citizens. My point is that the state must be justified in the acts it performs against individuals. They must be done for a reason and not as a tyrannical whim. Similarly the police must be justified in the actions they take to pursue criminals and stop crimes. They should not do everything they might wish, they should not send people to prison without trial, they should not gun someone down in the street because they think the person might deserve it.

The example does not need to be so extreme, the police should not come into your house and settle a dispute over who should take the rubbish out. In other words they have a jurisdiction. There are some things that they should be able to do, and some things they shouldn't.

Buffy is in a similar position. Her Slayer powers, her position as the Chosen One gives her the ability to kill things, but she does not automatically have the right to do so. I have the ability to buy a knife and kill those I believe harmful to society, but I don't have the right. However the series does represent that Buffy has the right to do some things, i.e. slay vampires (hence the show's name).

Her rights are limited, she has a jurisdiction. She can slay vampires and demons but she shouldn't slay humans. She can stop the end of the world but she shouldn't become involved in whether or not someone should get a parking ticket, unless it is in her capacity as a private citizen. She can stop a petty thief, because that's her right as a citizen, but she could not then judge him, condemn him and punish him. That would be for the courts.

Where do the state's powers come from? Its citizens and therefore its powers should only be exercised for the benefit of the citizen body as a whole.

Where do the Slayer's powers come from? Joss has deliberately avoided specifying from where the Slayer powers derive. Slayers are called, who by? "The powers that be," is not a satisfactory answer.

For all we know the Slayer powers could come from a demon, a demon who instead of using tainted lackeys like vampires instead chooses its champion from among mortals and uses that champion to defeat its rivals. Why does it allow Buffy to go around saving the world? Well maybe its motives is slightly different from its fellows. Maybe, for reasonable reasons of its own it doesn't particularly won't Hell on Earth, or at least only on its own terms. Maybe it has a monopoly on human flesh down there in the dark dimensions and doesn't want its customers getting a free nosebag. Maybe it owns all the property and is making a bomb on the rent. It's a demon, and as many have demonstrated before, they each have their own agenda and some, like Spike, kind of like the world as it is.

At any rate, we do not know what granted her powers. But it does not stop us from deciding what Buffy should and should not be able to do.