"If there was a free society without a state, wouldn't warlords take over?"
This is best answered by Robert P. Murphy's article, serendipitously named But Wouldn't Warlords Take Over?
In a little more detail, (1) with a state they already have, and you're paying tribute and obeying their edicts, and (2) distributed power is a great deterrent.
It must be realized that if there is one band with superior force, yeah, they're going to take over and possibly institute a state. "We must have a state to prevent the institution of a state" doesn't make much sense, however. Maybe "To prevent the institution of a worse state" but it's still just fear-mongering. The hope is that a free society will survive though education and distributed force—there are no prohibitions on building, importing, or trading arms—and be able to repel invasion from without and enslavement from within; but it is certainly possible it could fail… a guerrilla war might be fought to defeat the new tax-eaters; there are many possibilities.
This article about "forfeiture corridors" describes how drug dogs, whose alert is considered cause to search a vehicle or dwelling, are more likely to be alerting to a handler's cue than actual drugs, just so the police have an excuse to search vehicles and steal anything they find; and it can cost more than the stolen goods are worth to fight in court and get them back.
But oh, no, we must have the state or else gangs would rule the land, stopping travelers to rob them! (DBR)
Real-world Evidence Contradicts the Statist Narrative:
"The people of Michoacan, fed up with the fear and subjugation of the cartels and the inaction of the government, have taken a page from the American Revolution, organizing citizen militias that have now driven cartels from the region almost entirely. These militias have decided to no longer rely upon government intervention and have taken independent action outside of the forced authoritarian structure."
Of course, the Mexican government has retaliated against the citizens for "taking the law into their own hands," which is politi-speak for "you cannot compete with us!"