Let's start with the definition of free market. Free market just means a system wherein people voluntarily exchange their own goods in the absence of (state) coercion. For this to happen, there has to be acknowledged private property rights.
So from the above definition there cannot possibly be anything wrong with a free market because there is nothing wrong with voluntarily supplying and exchanging goods. In contrast to this natural social order of capitalism that needs no enforcement, socialism which is broadly defined by forced redistribution of property titles needs physical force of the state to succeed. This clearly violates natural law and therefore is immoral. The real problem is your view of the free market. You think it's about corporations exploiting laborers, ravaging the earth, and causing mass social inequality and poverty when in fact what we have is a neoliberal fascistic system.
Let's now go on to the monopoly problem. A firm gains profit and grows in a free market by giving value to the consumer. So if a firm grows very large, it just means it is successful in giving value to the consumer(and as a corollary employed lots of people and given high wages). So no one should really worry about corporate growth in a free market.
Monopolies by definition cannot exist in a free market because the free market by definition is characterized by free entry in the production of goods in addition to the fact that nobody is talented enough to have all the market share. Some anomalies to this are DeBeer and the New York Stock Exchange but the NYSE now takes government assistance. Monopolies can only exist because of government special privileges and corporate welfare and subsidies and most especially anti-trust laws. If you take a look at the anti-trust cases in America, it will be very clear to you that large firms are the ones that usually file them against their younger, smaller competitors. This is what creates a monopoly. US agribusiness like Monsanto receive 40% of their income through government subsidies. When you think about it and start from first principles and then integrate all the facts there is no doubt that what we have is NOT a free market.
Lastly, you said that monopolies would emerge because of mercenary recruitment and training of private armies. Go back to the definition of free market. It's a system where force is absent. So if a corporation hires a private army to attack its competitors, that is exactly the antithesis of a free market. In contrast, government itself functions entirely by means of force through taxation. Government defined as a territorial monopolist of law, order and ultimate decision making is the ULTIMATE PRIVATE ARMY and MONOPOLY!