While having a conversation about libertarianism in favour of or against, the following passage will be very relevant (Excerpt from Chapter 13 of Rothbard's 'Ethics of Liberty'):

'In the first place, it should be clear that the proportionate principle is a maximum, rather than a mandatory, punishment for the criminal. In the libertarian society, there are, as we have said, only two parties to a dispute or action at law: the victim, or plaintiff, and the alleged criminal, or defendant. It is the plaintiff that presses charges in the courts against the wrongdoer. In a libertarian world, there would be no crimes against an ill-defined “society,” and therefore no such person as a “district attorney” who decides on a charge and then presses those charges against an alleged criminal. The proportionality rule tells us how much punishment a plaintiff may exact from a convicted wrongdoer, and no more; it imposes the maximum limit on punishment that may be inflicted before the punisher himself becomes a criminal aggressor.'