I'm writing this, as I'm having a bitter argument with somebody. First, a music video.
At low population densities, humans are social, as mutual co-operation favours survival. Humans naturally form groups of about 150. At low population densities, anarchy works - for a while. However, inevitably, somebody starts cheating on somebody else. Cheating is beneficial to the cheater (until they get caught) and detrimental to the cheated. Cheaters may be physically punished, or evicted.
Nowadays, we have purpose-built places to which we evict cheaters. I know that modern-day prisons also contain people who have broken pointless laws.
As population density increases, unrest increases. There are power struggles & disputes.
In male-dominated societies, disputes are resolved by violence and murder.
In female-dominated societies, disputes are resolved by sex. N.B. Scenes of violence, murder and sex.
I know which society I'd prefer.
As population density increases, an increasing percentage of the population are antisocial.
EDIT: As population heterogeneity increases, an increasing percentage of the population are antisocial, due to group snobbery.
In the US, there is a high population density, also high heterogeneity. What do you think will happen if US citizens (who are free to carry guns) are left to run themselves? See 2012 - 2013 Violent Welfare Riots/ Looting spread across America - Gun confiscation in major cities. I predict rioting, looting and carnage.
In the UK, there is a very high population density, also high heterogeneity. What do you think will happen if UK citizens are left to run themselves? See UK - London Riots, BBC News: "Andy, we'll leave it there". I predict rioting & looting.
Rioting & looting are bad for business and are therefore not allowed.
I believe it's a sad fact of human nature that large, dense, human populations need to be controlled.