First recognize that _all_ property is private property. That's important context.

If you step on my property, I'll require you to have insurance.

If you murder someone, that person will (likely) have insurance that pays their estate in the event of their murder. Once insurance has paid the claim, they will attempt to collect funds from you. Your insurance will cover you for that. If there are disputes along the way here, private enforcement agencies will deal with that on their employers' behalf. The easiest, cheapest, least-violent (unnecessary violence is expensive, as we're seeing) way to handle those disputes will probably be through arbitration, which the enforcement agencies will mostly align on in order to economize and work efficiently. No where has coercion been introduced.

If you _don't_ have proper insurance, you are unlikely to be allowed on most private property. So to the extent that you'd like to operate normally in society, you'll carry the level of insurance that allows you to do so.

If you're a truly antisocial, masochistic criminal who opts to carry no insurance and attempts to go on a crime spree, the first bit of private property you step on will find you in violation of their "entry requirements" and will use force to repel you, and nobody in the voluntary society will see that as unjustified, as you willfully violated a contract that warned against your subsequent behavior.

Given that nearly every (voluntary) contract at any level will have large penalties for murder, it would essentially be taken as a universal that "murder is 'illegal' ". That's a spontaneous, bottom-up "rule" that doesn't require a State to prescribe. Freely-associating individuals can arrive at that.