Organized crime, drug dealers, etc. often kill people for small offenses, "honor" crimes, ratting out, snooping or just for fun. Haven't you seen any mafia/yakuza movies? Real life is much, much worse.
Drug dealers in the form you're talking about can only exist when there is drug prohibition, and it's highly unlikely that a society based on polycentric law would prohibit drugs. The economics of a lot of organized crime also can be explained government.
Even in a centralized state, the best, most efficient and reliable way to distribute illegal items would be total decentralization. Huge crime rings still exist though: where there is the opportunity to monopolize a resource, people will do it. That will only be exaggerated in your imagined anarcho-capitalist state. Even resources like water, energy, raw materials would be subject to monopolization, and very likely enforced through weaponry, not law. Can you see where that's going?
Suppose I kill someone. Now you'll turn to a private law company to judge/punish me. I then proceed to kill that company's employees. Will you now invoke a third private law provider in their name? Who will pay for it? Even if all the law providers were mutually insured, in this arrangement, as long as I can overpower each private entity, there is no effective law.
> That will only be exaggerated in your imagined anarcho-capitalist state.
Why? This is an unsubstantiated claim, and I don't see why it's any truer than claiming that competition in the retail industry will be worse than a government-owned retail monopoly.
> Suppose I kill someone. Now you'll turn to a private law company to judge/punish me. I then proceed to kill that company's employees. Will you now invoke a third private law provider in their name? Who will pay for it?
All you're arguing here is that my proposition would not be a perfect utopian society, which I would never be foolish enough to claim. It's no different than asking "What if I kill someone, then kill the cops that show up, then kill every other law enforcement agency that shows up, then kill the entire national military?"