The Libertarian View: Police - The Problem and Solution. Rothbard & Brennan

In short, the problem with police is they are (a) coercively funded, any organization who's 'customers' can't opt out will get worse service than they otherwise would; (b) a monopoly, one cannot start up a competing police agency with the state, and (c) they claim the 'special immunity' of having the right to forcibly tell others what to do because politicians told them to- they can kidnap people for disobeying the laws written by government officials widely accepted to be corrupt and dishonest.